diff --git a/bind/bind_test.go b/bind/bind_test.go
index 5313b4d..eb75115 100644
--- a/bind/bind_test.go
+++ b/bind/bind_test.go
@@ -30,6 +30,7 @@ var tests = []string{
 	"testdata/interfaces.go",
 	"testdata/issue10788.go",
 	"testdata/issue12328.go",
+	"testdata/issue12403.go",
 	"testdata/try.go",
 }
 
diff --git a/bind/genobjc.go b/bind/genobjc.go
index 00b822c..fcc7350 100644
--- a/bind/genobjc.go
+++ b/bind/genobjc.go
@@ -597,7 +597,7 @@ func (g *objcGen) genInterfaceMethodProxy(obj *types.TypeName, s *funcSummary) {
 	if s.returnsVal() { // len(s.retParams) == 1 && s.retParams[0] != error
 		p := s.retParams[0]
 		if stype := g.seqType(p.typ); stype == "Ref" {
-			g.Printf("if [(id<NSObject>)(returnVal) isKindOfClass:[%s class]]) {\n", g.refTypeBase(p.typ))
+			g.Printf("if ([(id<NSObject>)(returnVal) isKindOfClass:[%s class]]) {\n", g.refTypeBase(p.typ))
 			g.Indent()
 			g.Printf("id<goSeqRefInterface>retVal_proxy = (id<goSeqRefInterface>)(returnVal);\n")
 			g.Printf("go_seq_writeRef(out, retVal_proxy.ref);\n")
@@ -633,9 +633,9 @@ func (g *objcGen) genInterfaceMethodProxy(obj *types.TypeName, s *funcSummary) {
 			g.Printf("go_seq_writeUTF8(out, %sDesc);\n", p.name)
 			g.Outdent()
 			g.Printf("}\n")
-		} else if seqTyp := g.seqType(p.typ); seqTyp != "Ref" {
+		} else if seqTyp := g.seqType(p.typ); seqTyp == "Ref" {
 			// TODO(hyangah): NULL.
-			g.Printf("if [(id<NSObject>)(%s) isKindOfClass:[%s class]]) {\n", p.name, g.refTypeBase(p.typ))
+			g.Printf("if ([(id<NSObject>)(%s) isKindOfClass:[%s class]]) {\n", p.name, g.refTypeBase(p.typ))
 			g.Indent()
 			g.Printf("id<goSeqRefInterface>%[1]s_proxy = (id<goSeqRefInterface>)(%[1]s);\n", p.name)
 			g.Printf("go_seq_writeRef(out, %s_proxy.ref);\n", p.name)
@@ -735,6 +735,7 @@ func (g *objcGen) refTypeBase(typ types.Type) string {
 	}
 
 	// fallback to whatever objcType returns. This must not happen.
+	panic(fmt.Sprintf("wtf: %+T", typ))
 	return g.objcType(typ)
 }
 
diff --git a/bind/objc/SeqTest.m b/bind/objc/SeqTest.m
index 9de0dd9..2607a67 100644
--- a/bind/objc/SeqTest.m
+++ b/bind/objc/SeqTest.m
@@ -107,6 +107,18 @@ static int numI = 0;
 }
 @synthesize value;
 
+- (BOOL)StringError:(NSString *)s
+              ret0_:(NSString **)ret0_
+              error:(NSError **)error {
+  if ([s isEqualTo:@"number"]) {
+    if (ret0_ != NULL) {
+      *ret0_ = @"OK";
+    }
+    return true;
+  }
+  return false;
+}
+
 - (BOOL)Error:(BOOL)triggerError error:(NSError **)error {
   if (!triggerError) {
     return YES;
@@ -120,6 +132,7 @@ static int numI = 0;
 - (int64_t)Times:(int32_t)v {
   return v * value;
 }
+
 - (void)dealloc {
   if (self.value == 0) {
     numI++;
@@ -179,6 +192,28 @@ void testIssue12307() {
   }
 }
 
+void testIssue12403() {
+  Number *num = [[Number alloc] init];
+  num.value = 1024;
+
+  NSString *ret;
+  NSError *error;
+  if (GoTestpkgCallIStringError(num, @"alphabet", &ret, &error) == YES) {
+    ERROR(
+        @"GoTestpkgCallIStringError(Number, 'alphabet') succeeded; want error");
+  }
+  NSError *error2;
+  if (GoTestpkgCallIStringError(num, @"number", &ret, &error2) == NO) {
+    ERROR(
+        @"GoTestpkgCallIStringError(Number, 'number') failed(%@); want success",
+        error2);
+  } else if (![ret isEqualTo:@"OK"]) {
+    ERROR(@"GoTestpkgCallIStringError(Number, 'number') returned unexpected "
+          @"results %@",
+          ret);
+  }
+}
+
 // Invokes functions and object methods defined in Testpkg.h.
 //
 // TODO(hyangah): apply testing framework (e.g. XCTestCase)
diff --git a/bind/objc/testpkg/go_testpkg/go_testpkg.go b/bind/objc/testpkg/go_testpkg/go_testpkg.go
index 86d3beb..45a23da 100644
--- a/bind/objc/testpkg/go_testpkg/go_testpkg.go
+++ b/bind/objc/testpkg/go_testpkg/go_testpkg.go
@@ -33,6 +33,24 @@ func proxy_CallIError(out, in *seq.Buffer) {
 	}
 }
 
+func proxy_CallIStringError(out, in *seq.Buffer) {
+	var param_i testpkg.I
+	param_i_ref := in.ReadRef()
+	if param_i_ref.Num < 0 { // go object
+		param_i = param_i_ref.Get().(testpkg.I)
+	} else { // foreign object
+		param_i = (*proxyI)(param_i_ref)
+	}
+	param_s := in.ReadString()
+	res, err := testpkg.CallIStringError(param_i, param_s)
+	out.WriteString(res)
+	if err == nil {
+		out.WriteString("")
+	} else {
+		out.WriteString(err.Error())
+	}
+}
+
 func proxy_CallSSum(out, in *seq.Buffer) {
 	// Must be a Go object
 	param_s_ref := in.ReadRef()
@@ -63,9 +81,10 @@ func proxy_Hi(out, in *seq.Buffer) {
 }
 
 const (
-	proxyI_Descriptor = "go.testpkg.I"
-	proxyI_Error_Code = 0x10a
-	proxyI_Times_Code = 0x20a
+	proxyI_Descriptor       = "go.testpkg.I"
+	proxyI_Error_Code       = 0x10a
+	proxyI_StringError_Code = 0x20a
+	proxyI_Times_Code       = 0x30a
 )
 
 func proxyI_Error(out, in *seq.Buffer) {
@@ -80,6 +99,19 @@ func proxyI_Error(out, in *seq.Buffer) {
 	}
 }
 
+func proxyI_StringError(out, in *seq.Buffer) {
+	ref := in.ReadRef()
+	v := ref.Get().(testpkg.I)
+	param_s := in.ReadString()
+	res, err := v.StringError(param_s)
+	out.WriteString(res)
+	if err == nil {
+		out.WriteString("")
+	} else {
+		out.WriteString(err.Error())
+	}
+}
+
 func proxyI_Times(out, in *seq.Buffer) {
 	ref := in.ReadRef()
 	v := ref.Get().(testpkg.I)
@@ -90,6 +122,7 @@ func proxyI_Times(out, in *seq.Buffer) {
 
 func init() {
 	seq.Register(proxyI_Descriptor, proxyI_Error_Code, proxyI_Error)
+	seq.Register(proxyI_Descriptor, proxyI_StringError_Code, proxyI_StringError)
 	seq.Register(proxyI_Descriptor, proxyI_Times_Code, proxyI_Times)
 }
 
@@ -103,6 +136,15 @@ func (p *proxyI) Error(triggerError bool) error {
 	return res_0
 }
 
+func (p *proxyI) StringError(s string) (string, error) {
+	in := new(seq.Buffer)
+	in.WriteString(s)
+	out := seq.Transact((*seq.Ref)(p), "go.testpkg.I", proxyI_StringError_Code, in)
+	res_0 := out.ReadString()
+	res_1 := out.ReadError()
+	return res_0, res_1
+}
+
 func (p *proxyI) Times(v int32) int64 {
 	in := new(seq.Buffer)
 	in.WriteInt32(v)
@@ -285,18 +327,19 @@ func proxy_UnregisterI(out, in *seq.Buffer) {
 func init() {
 	seq.Register("testpkg", 1, proxy_BytesAppend)
 	seq.Register("testpkg", 2, proxy_CallIError)
-	seq.Register("testpkg", 3, proxy_CallSSum)
-	seq.Register("testpkg", 4, proxy_CollectS)
-	seq.Register("testpkg", 5, proxy_GC)
-	seq.Register("testpkg", 6, proxy_Hello)
-	seq.Register("testpkg", 7, proxy_Hi)
-	seq.Register("testpkg", 8, proxy_Int)
-	seq.Register("testpkg", 9, proxy_Multiply)
-	seq.Register("testpkg", 10, proxy_NewI)
-	seq.Register("testpkg", 11, proxy_NewNode)
-	seq.Register("testpkg", 12, proxy_NewS)
-	seq.Register("testpkg", 13, proxy_RegisterI)
-	seq.Register("testpkg", 14, proxy_ReturnsError)
-	seq.Register("testpkg", 15, proxy_Sum)
-	seq.Register("testpkg", 16, proxy_UnregisterI)
+	seq.Register("testpkg", 3, proxy_CallIStringError)
+	seq.Register("testpkg", 4, proxy_CallSSum)
+	seq.Register("testpkg", 5, proxy_CollectS)
+	seq.Register("testpkg", 6, proxy_GC)
+	seq.Register("testpkg", 7, proxy_Hello)
+	seq.Register("testpkg", 8, proxy_Hi)
+	seq.Register("testpkg", 9, proxy_Int)
+	seq.Register("testpkg", 10, proxy_Multiply)
+	seq.Register("testpkg", 11, proxy_NewI)
+	seq.Register("testpkg", 12, proxy_NewNode)
+	seq.Register("testpkg", 13, proxy_NewS)
+	seq.Register("testpkg", 14, proxy_RegisterI)
+	seq.Register("testpkg", 15, proxy_ReturnsError)
+	seq.Register("testpkg", 16, proxy_Sum)
+	seq.Register("testpkg", 17, proxy_UnregisterI)
 }
diff --git a/bind/objc/testpkg/objc_testpkg/GoTestpkg.h b/bind/objc/testpkg/objc_testpkg/GoTestpkg.h
index 141469d..224a18d 100644
--- a/bind/objc/testpkg/objc_testpkg/GoTestpkg.h
+++ b/bind/objc/testpkg/objc_testpkg/GoTestpkg.h
@@ -14,6 +14,7 @@
 
 @protocol GoTestpkgI
 - (BOOL)Error:(BOOL)triggerError error:(NSError**)error;
+- (BOOL)StringError:(NSString*)s ret0_:(NSString**)ret0_ error:(NSError**)error;
 - (int64_t)Times:(int32_t)v;
 @end
 
@@ -45,6 +46,8 @@ FOUNDATION_EXPORT NSData* GoTestpkgBytesAppend(NSData* a, NSData* b);
 
 FOUNDATION_EXPORT BOOL GoTestpkgCallIError(id<GoTestpkgI> i, BOOL triggerError, NSError** error);
 
+FOUNDATION_EXPORT BOOL GoTestpkgCallIStringError(id<GoTestpkgI> i, NSString* s, NSString** ret0_, NSError** error);
+
 FOUNDATION_EXPORT double GoTestpkgCallSSum(GoTestpkgS* s);
 
 FOUNDATION_EXPORT int GoTestpkgCollectS(int want, int timeoutSec);
diff --git a/bind/objc/testpkg/objc_testpkg/GoTestpkg.m b/bind/objc/testpkg/objc_testpkg/GoTestpkg.m
index ef40f24..345ef21 100644
--- a/bind/objc/testpkg/objc_testpkg/GoTestpkg.m
+++ b/bind/objc/testpkg/objc_testpkg/GoTestpkg.m
@@ -17,24 +17,26 @@ static NSString* errDomain = @"go.golang.org/x/mobile/bind/objc/testpkg";
 
 #define _CALL_BytesAppend_ 1
 #define _CALL_CallIError_ 2
-#define _CALL_CallSSum_ 3
-#define _CALL_CollectS_ 4
-#define _CALL_GC_ 5
-#define _CALL_Hello_ 6
-#define _CALL_Hi_ 7
-#define _CALL_Int_ 8
-#define _CALL_Multiply_ 9
-#define _CALL_NewI_ 10
-#define _CALL_NewNode_ 11
-#define _CALL_NewS_ 12
-#define _CALL_RegisterI_ 13
-#define _CALL_ReturnsError_ 14
-#define _CALL_Sum_ 15
-#define _CALL_UnregisterI_ 16
+#define _CALL_CallIStringError_ 3
+#define _CALL_CallSSum_ 4
+#define _CALL_CollectS_ 5
+#define _CALL_GC_ 6
+#define _CALL_Hello_ 7
+#define _CALL_Hi_ 8
+#define _CALL_Int_ 9
+#define _CALL_Multiply_ 10
+#define _CALL_NewI_ 11
+#define _CALL_NewNode_ 12
+#define _CALL_NewS_ 13
+#define _CALL_RegisterI_ 14
+#define _CALL_ReturnsError_ 15
+#define _CALL_Sum_ 16
+#define _CALL_UnregisterI_ 17
 
 #define _GO_testpkg_I_DESCRIPTOR_ "go.testpkg.I"
 #define _GO_testpkg_I_Error_ (0x10a)
-#define _GO_testpkg_I_Times_ (0x20a)
+#define _GO_testpkg_I_StringError_ (0x20a)
+#define _GO_testpkg_I_Times_ (0x30a)
 
 @interface GoTestpkgI : NSObject <GoTestpkgI> {
 }
@@ -42,6 +44,7 @@ static NSString* errDomain = @"go.golang.org/x/mobile/bind/objc/testpkg";
 
 - (id)initWithRef:(id)ref;
 - (BOOL)Error:(BOOL)triggerError error:(NSError**)error;
+- (BOOL)StringError:(NSString*)s ret0_:(NSString**)ret0_ error:(NSError**)error;
 - (int64_t)Times:(int32_t)v;
 @end
 
@@ -71,6 +74,27 @@ static NSString* errDomain = @"go.golang.org/x/mobile/bind/objc/testpkg";
 	return ([_error length] == 0);
 }
 
+- (BOOL)StringError:(NSString*)s ret0_:(NSString**)ret0_ error:(NSError**)error {
+	GoSeq in_ = {};
+	GoSeq out_ = {};
+	go_seq_writeRef(&in_, self.ref);
+	go_seq_writeUTF8(&in_, s);
+	go_seq_send(_GO_testpkg_I_DESCRIPTOR_, _GO_testpkg_I_StringError_, &in_, &out_);
+	NSString* ret0__val = go_seq_readUTF8(&out_);
+	if (ret0_ != NULL) {
+		*ret0_ = ret0__val;
+	}
+	NSString* _error = go_seq_readUTF8(&out_);
+	if ([_error length] != 0 && error != nil) {
+		NSMutableDictionary* details = [NSMutableDictionary dictionary];
+		[details setValue:_error forKey:NSLocalizedDescriptionKey];
+		*error = [NSError errorWithDomain:errDomain code:1 userInfo:details];
+	}
+	go_seq_free(&in_);
+	go_seq_free(&out_);
+	return ([_error length] == 0);
+}
+
 - (int64_t)Times:(int32_t)v {
 	GoSeq in_ = {};
 	GoSeq out_ = {};
@@ -102,6 +126,23 @@ static void proxyGoTestpkgI(id obj, int code, GoSeq* in, GoSeq* out) {
 			go_seq_writeUTF8(out, errorDesc);
 		}
 	} break;
+	case _GO_testpkg_I_StringError_: {
+		id<GoTestpkgI> o = (id<GoTestpkgI>)(obj);
+		NSString* s = go_seq_readUTF8(in);
+		NSString* ret0_;
+		NSError* error = NULL;
+		BOOL returnVal = [o StringError:s ret0_:&ret0_ error:&error];
+		go_seq_writeUTF8(out, ret0_);
+		if (returnVal) {
+			go_seq_writeUTF8(out, NULL);
+		} else {
+			NSString* errorDesc = [error localizedDescription];
+			if (errorDesc == NULL || errorDesc.length == 0) {
+				errorDesc = @"gobind: unknown error";
+			}
+			go_seq_writeUTF8(out, errorDesc);
+		}
+	} break;
 	case _GO_testpkg_I_Times_: {
 		id<GoTestpkgI> o = (id<GoTestpkgI>)(obj);
 		int32_t v = go_seq_readInt32(in);
@@ -291,6 +332,32 @@ BOOL GoTestpkgCallIError(id<GoTestpkgI> i, BOOL triggerError, NSError** error) {
 	return ([_error length] == 0);
 }
 
+BOOL GoTestpkgCallIStringError(id<GoTestpkgI> i, NSString* s, NSString** ret0_, NSError** error) {
+	GoSeq in_ = {};
+	GoSeq out_ = {};
+	if ([(id<NSObject>)(i) isKindOfClass:[GoTestpkgI class]]) {
+		id<goSeqRefInterface> i_proxy = (id<goSeqRefInterface>)(i);
+		go_seq_writeRef(&in_, i_proxy.ref);
+	} else {
+		go_seq_writeObjcRef(&in_, i);
+	}
+	go_seq_writeUTF8(&in_, s);
+	go_seq_send(_DESCRIPTOR_, _CALL_CallIStringError_, &in_, &out_);
+	NSString* ret0__val = go_seq_readUTF8(&out_);
+	if (ret0_ != NULL) {
+		*ret0_ = ret0__val;
+	}
+	NSString* _error = go_seq_readUTF8(&out_);
+	if ([_error length] != 0 && error != nil) {
+		NSMutableDictionary* details = [NSMutableDictionary dictionary];
+		[details setValue:_error forKey:NSLocalizedDescriptionKey];
+		*error = [NSError errorWithDomain:errDomain code:1 userInfo:details];
+	}
+	go_seq_free(&in_);
+	go_seq_free(&out_);
+	return ([_error length] == 0);
+}
+
 double GoTestpkgCallSSum(GoTestpkgS* s) {
 	GoSeq in_ = {};
 	GoSeq out_ = {};
diff --git a/bind/objc/testpkg/testpkg.go b/bind/objc/testpkg/testpkg.go
index 7238f47..d5c5218 100644
--- a/bind/objc/testpkg/testpkg.go
+++ b/bind/objc/testpkg/testpkg.go
@@ -17,6 +17,8 @@ import (
 type I interface {
 	Times(v int32) int64
 	Error(triggerError bool) error
+
+	StringError(s string) (string, error)
 }
 
 type myI struct{}
@@ -32,10 +34,18 @@ func (i *myI) Error(e bool) error {
 	return nil
 }
 
+func (i *myI) StringError(s string) (string, error) {
+	return s, nil
+}
+
 func CallIError(i I, triggerError bool) error {
 	return i.Error(triggerError)
 }
 
+func CallIStringError(i I, s string) (string, error) {
+	return i.StringError(s)
+}
+
 func NewI() I {
 	return &myI{}
 }
diff --git a/bind/testdata/interfaces.go.golden b/bind/testdata/interfaces.go.golden
index fc2dc68..a3fa216 100644
--- a/bind/testdata/interfaces.go.golden
+++ b/bind/testdata/interfaces.go.golden
@@ -146,8 +146,6 @@ func (p *proxyI3) F() interfaces.I1 {
 	res_0_ref := out.ReadRef()
 	if res_0_ref.Num < 0 { // go object
 		res_0 = res_0_ref.Get().(interfaces.I1)
-	} else { // foreign object
-		res_0 = (*proxyI1)(res_0_ref)
 	}
 	return res_0
 }
diff --git a/bind/testdata/interfaces.objc.h.golden b/bind/testdata/interfaces.objc.h.golden
index 7f277d1..794deb0 100644
--- a/bind/testdata/interfaces.objc.h.golden
+++ b/bind/testdata/interfaces.objc.h.golden
@@ -8,6 +8,10 @@
 
 #include <Foundation/Foundation.h>
 
+@class GoInterfacesI1;
+
+@class GoInterfacesI2;
+
 @protocol GoInterfacesError
 - (BOOL)Err:(NSError**)error;
 @end
@@ -35,7 +39,7 @@
 
 
 @protocol GoInterfacesI3
-- (id<GoInterfacesI1>)F;
+- (GoInterfacesI1*)F;
 @end
 
 @protocol GoInterfacesWithParam
diff --git a/bind/testdata/interfaces.objc.m.golden b/bind/testdata/interfaces.objc.m.golden
index 06fc493..72dea5f 100644
--- a/bind/testdata/interfaces.objc.m.golden
+++ b/bind/testdata/interfaces.objc.m.golden
@@ -179,7 +179,7 @@ static void proxyGoInterfacesI(id obj, int code, GoSeq* in, GoSeq* out) {
 @property(strong, readonly) id ref;
 
 - (id)initWithRef:(id)ref;
-- (id<GoInterfacesI1>)F;
+- (GoInterfacesI1*)F;
 @end
 
 @implementation GoInterfacesI3 {
@@ -191,13 +191,13 @@ static void proxyGoInterfacesI(id obj, int code, GoSeq* in, GoSeq* out) {
 	return self;
 }
 
-- (id<GoInterfacesI1>)F {
+- (GoInterfacesI1*)F {
 	GoSeq in_ = {};
 	GoSeq out_ = {};
 	go_seq_writeRef(&in_, self.ref);
 	go_seq_send(_GO_interfaces_I3_DESCRIPTOR_, _GO_interfaces_I3_F_, &in_, &out_);
 	GoSeqRef* ret0__ref = go_seq_readRef(&out_);
-	id<GoInterfacesI1> ret0_ = ret0__ref.obj;
+	GoInterfacesI1* ret0_ = ret0__ref.obj;
 	if (ret0_ == NULL) {
 		ret0_ = [[GoInterfacesI1 alloc] initWithRef:ret0__ref];
 	}
@@ -212,8 +212,8 @@ static void proxyGoInterfacesI3(id obj, int code, GoSeq* in, GoSeq* out) {
 	switch (code) {
 	case _GO_interfaces_I3_F_: {
 		id<GoInterfacesI3> o = (id<GoInterfacesI3>)(obj);
-		id<GoInterfacesI1> returnVal = [o F];
-		if [(id<NSObject>)(returnVal) isKindOfClass:[GoInterfacesI1 class]]) {
+		GoInterfacesI1* returnVal = [o F];
+		if ([(id<NSObject>)(returnVal) isKindOfClass:[GoInterfacesI1 class]]) {
 			id<goSeqRefInterface>retVal_proxy = (id<goSeqRefInterface>)(returnVal);
 			go_seq_writeRef(out, retVal_proxy.ref);
 		} else {
diff --git a/bind/testdata/issue12403.go b/bind/testdata/issue12403.go
new file mode 100644
index 0000000..bf55567
--- /dev/null
+++ b/bind/testdata/issue12403.go
@@ -0,0 +1,6 @@
+package issue12403
+
+type Parsable interface {
+	FromJSON(jstr string) string
+	ToJSON() (string, error)
+}
diff --git a/bind/testdata/issue12403.go.golden b/bind/testdata/issue12403.go.golden
new file mode 100644
index 0000000..7708d5c
--- /dev/null
+++ b/bind/testdata/issue12403.go.golden
@@ -0,0 +1,59 @@
+// Package go_issue12403 is an autogenerated binder stub for package issue12403.
+//   gobind -lang=go issue12403
+//
+// File is generated by gobind. Do not edit.
+package go_issue12403
+
+import (
+	"golang.org/x/mobile/bind/seq"
+	"issue12403"
+)
+
+const (
+	proxyParsable_Descriptor    = "go.issue12403.Parsable"
+	proxyParsable_FromJSON_Code = 0x10a
+	proxyParsable_ToJSON_Code   = 0x20a
+)
+
+func proxyParsable_FromJSON(out, in *seq.Buffer) {
+	ref := in.ReadRef()
+	v := ref.Get().(issue12403.Parsable)
+	param_jstr := in.ReadString()
+	res := v.FromJSON(param_jstr)
+	out.WriteString(res)
+}
+
+func proxyParsable_ToJSON(out, in *seq.Buffer) {
+	ref := in.ReadRef()
+	v := ref.Get().(issue12403.Parsable)
+	res, err := v.ToJSON()
+	out.WriteString(res)
+	if err == nil {
+		out.WriteString("")
+	} else {
+		out.WriteString(err.Error())
+	}
+}
+
+func init() {
+	seq.Register(proxyParsable_Descriptor, proxyParsable_FromJSON_Code, proxyParsable_FromJSON)
+	seq.Register(proxyParsable_Descriptor, proxyParsable_ToJSON_Code, proxyParsable_ToJSON)
+}
+
+type proxyParsable seq.Ref
+
+func (p *proxyParsable) FromJSON(jstr string) string {
+	in := new(seq.Buffer)
+	in.WriteString(jstr)
+	out := seq.Transact((*seq.Ref)(p), "go.issue12403.Parsable", proxyParsable_FromJSON_Code, in)
+	res_0 := out.ReadString()
+	return res_0
+}
+
+func (p *proxyParsable) ToJSON() (string, error) {
+	in := new(seq.Buffer)
+	out := seq.Transact((*seq.Ref)(p), "go.issue12403.Parsable", proxyParsable_ToJSON_Code, in)
+	res_0 := out.ReadString()
+	res_1 := out.ReadError()
+	return res_0, res_1
+}
diff --git a/bind/testdata/issue12403.java.golden b/bind/testdata/issue12403.java.golden
new file mode 100644
index 0000000..e98508e
--- /dev/null
+++ b/bind/testdata/issue12403.java.golden
@@ -0,0 +1,98 @@
+// Java class go.issue12403.Issue12403 is a proxy for talking to a Go program.
+//   gobind -lang=java issue12403
+//
+// File is generated by gobind. Do not edit.
+package go.issue12403;
+
+import go.Seq;
+
+public abstract class Issue12403 {
+    private Issue12403() {} // uninstantiable
+    
+    public interface Parsable extends go.Seq.Object {
+        public String FromJSON(String jstr);
+        
+        public String ToJSON() throws Exception;
+        
+        public static abstract class Stub implements Parsable {
+            static final String DESCRIPTOR = "go.issue12403.Parsable";
+            
+            private final go.Seq.Ref ref;
+            public Stub() {
+                ref = go.Seq.createRef(this);
+            }
+            
+            public go.Seq.Ref ref() { return ref; }
+            
+            public void call(int code, go.Seq in, go.Seq out) {
+                switch (code) {
+                case Proxy.CALL_FromJSON: {
+                    String param_jstr;
+                    param_jstr = in.readString();
+                    String result = this.FromJSON(param_jstr);
+                    out.writeString(result);
+                    return;
+                }
+                case Proxy.CALL_ToJSON: {
+                    try {
+                        String result = this.ToJSON();
+                        out.writeString(result);
+                        out.writeString(null);
+                    } catch (Exception e) {
+                        String result = null;
+                        out.writeString(result);
+                        out.writeString(e.getMessage());
+                    }
+                    return;
+                }
+                default:
+                    throw new RuntimeException("unknown code: "+ code);
+                }
+            }
+        }
+        
+        static final class Proxy implements Parsable {
+            static final String DESCRIPTOR = Stub.DESCRIPTOR;
+        
+            private go.Seq.Ref ref;
+        
+            Proxy(go.Seq.Ref ref) { this.ref = ref; }
+        
+            public go.Seq.Ref ref() { return ref; }
+        
+            public void call(int code, go.Seq in, go.Seq out) {
+                throw new RuntimeException("cycle: cannot call proxy");
+            }
+        
+            public String FromJSON(String jstr) {
+                go.Seq _in = new go.Seq();
+                go.Seq _out = new go.Seq();
+                String _result;
+                _in.writeRef(ref);
+                _in.writeString(jstr);
+                Seq.send(DESCRIPTOR, CALL_FromJSON, _in, _out);
+                _result = _out.readString();
+                return _result;
+            }
+            
+            public String ToJSON() throws Exception {
+                go.Seq _in = new go.Seq();
+                go.Seq _out = new go.Seq();
+                String _result;
+                _in.writeRef(ref);
+                Seq.send(DESCRIPTOR, CALL_ToJSON, _in, _out);
+                _result = _out.readString();
+                String _err = _out.readString();
+                if (_err != null) {
+                    throw new Exception(_err);
+                }
+                return _result;
+            }
+            
+            static final int CALL_FromJSON = 0x10a;
+            static final int CALL_ToJSON = 0x20a;
+        }
+    }
+    
+    private static final String DESCRIPTOR = "issue12403";
+}
diff --git a/bind/testdata/issue12403.objc.h.golden b/bind/testdata/issue12403.objc.h.golden
new file mode 100644
index 0000000..391e988
--- /dev/null
+++ b/bind/testdata/issue12403.objc.h.golden
@@ -0,0 +1,16 @@
+// Objective-C API for talking to issue12403 Go package.
+//   gobind -lang=objc issue12403
+//
+// File is generated by gobind. Do not edit.
+
+#ifndef __GoIssue12403_H__
+#define __GoIssue12403_H__
+
+#include <Foundation/Foundation.h>
+
+@protocol GoIssue12403Parsable
+- (NSString*)FromJSON:(NSString*)jstr;
+- (BOOL)ToJSON:(NSString**)ret0_ error:(NSError**)error;
+@end
+
+#endif
diff --git a/bind/testdata/issue12403.objc.m.golden b/bind/testdata/issue12403.objc.m.golden
new file mode 100644
index 0000000..c95c045
--- /dev/null
+++ b/bind/testdata/issue12403.objc.m.golden
@@ -0,0 +1,106 @@
+// Objective-C API for talking to issue12403 Go package.
+//   gobind -lang=objc issue12403
+//
+// File is generated by gobind. Do not edit.
+
+#include "GoIssue12403.h"
+#include <Foundation/Foundation.h>
+#include "seq.h"
+
+static NSString* errDomain = @"go.issue12403";
+
+@protocol goSeqRefInterface
+-(GoSeqRef*) ref;
+@end
+
+#define _DESCRIPTOR_ "issue12403"
+
+
+#define _GO_issue12403_Parsable_DESCRIPTOR_ "go.issue12403.Parsable"
+#define _GO_issue12403_Parsable_FromJSON_ (0x10a)
+#define _GO_issue12403_Parsable_ToJSON_ (0x20a)
+
+@interface GoIssue12403Parsable : NSObject <GoIssue12403Parsable> {
+}
+@property(strong, readonly) id ref;
+
+- (id)initWithRef:(id)ref;
+- (NSString*)FromJSON:(NSString*)jstr;
+- (BOOL)ToJSON:(NSString**)ret0_ error:(NSError**)error;
+@end
+
+@implementation GoIssue12403Parsable {
+}
+
+- (id)initWithRef:(id)ref {
+	self = [super init];
+	if (self) { _ref = ref; }
+	return self;
+}
+
+- (NSString*)FromJSON:(NSString*)jstr {
+	GoSeq in_ = {};
+	GoSeq out_ = {};
+	go_seq_writeRef(&in_, self.ref);
+	go_seq_writeUTF8(&in_, jstr);
+	go_seq_send(_GO_issue12403_Parsable_DESCRIPTOR_, _GO_issue12403_Parsable_FromJSON_, &in_, &out_);
+	NSString* ret0_ = go_seq_readUTF8(&out_);
+	go_seq_free(&in_);
+	go_seq_free(&out_);
+	return ret0_;
+}
+
+- (BOOL)ToJSON:(NSString**)ret0_ error:(NSError**)error {
+	GoSeq in_ = {};
+	GoSeq out_ = {};
+	go_seq_writeRef(&in_, self.ref);
+	go_seq_send(_GO_issue12403_Parsable_DESCRIPTOR_, _GO_issue12403_Parsable_ToJSON_, &in_, &out_);
+	NSString* ret0__val = go_seq_readUTF8(&out_);
+	if (ret0_ != NULL) {
+		*ret0_ = ret0__val;
+	}
+	NSString* _error = go_seq_readUTF8(&out_);
+	if ([_error length] != 0 && error != nil) {
+		NSMutableDictionary* details = [NSMutableDictionary dictionary];
+		[details setValue:_error forKey:NSLocalizedDescriptionKey];
+		*error = [NSError errorWithDomain:errDomain code:1 userInfo:details];
+	}
+	go_seq_free(&in_);
+	go_seq_free(&out_);
+	return ([_error length] == 0);
+}
+
+@end
+
+static void proxyGoIssue12403Parsable(id obj, int code, GoSeq* in, GoSeq* out) {
+	switch (code) {
+	case _GO_issue12403_Parsable_FromJSON_: {
+		id<GoIssue12403Parsable> o = (id<GoIssue12403Parsable>)(obj);
+		NSString* jstr = go_seq_readUTF8(in);
+		NSString* returnVal = [o FromJSON:jstr];
+		go_seq_writeUTF8(out, returnVal);
+	} break;
+	case _GO_issue12403_Parsable_ToJSON_: {
+		id<GoIssue12403Parsable> o = (id<GoIssue12403Parsable>)(obj);
+		NSString* ret0_;
+		NSError* error = NULL;
+		BOOL returnVal = [o ToJSON:&ret0_ error:&error];
+		go_seq_writeUTF8(out, ret0_);
+		if (returnVal) {
+			go_seq_writeUTF8(out, NULL);
+		} else {
+			NSString* errorDesc = [error localizedDescription];
+			if (errorDesc == NULL || errorDesc.length == 0) {
+				errorDesc = @"gobind: unknown error";
+			}
+			go_seq_writeUTF8(out, errorDesc);
+		}
+	} break;
+	default:
+		NSLog(@"unknown code %x for _GO_issue12403_Parsable_DESCRIPTOR_", code);
+	}
+}
+
+__attribute__((constructor)) static void init() {
+	go_seq_register_proxy("go.issue12403.Parsable", proxyGoIssue12403Parsable);
+}