diff --git a/rpc/args.go b/rpc/args.go
index 9f99456e6..7694a3d3f 100644
--- a/rpc/args.go
+++ b/rpc/args.go
@@ -535,11 +535,11 @@ func (args *BlockNumIndexArgs) UnmarshalJSON(b []byte) (err error) {
 		return err
 	}
 
-	arg1, ok := obj[1].(string)
-	if !ok {
-		return NewInvalidTypeError("index", "not a string")
+	var arg1 *big.Int
+	if arg1, err = numString(obj[1]); err != nil {
+		return err
 	}
-	args.Index = common.Big(arg1).Int64()
+	args.Index = arg1.Int64()
 
 	return nil
 }
diff --git a/rpc/args_test.go b/rpc/args_test.go
index e78981a41..9e0cf179f 100644
--- a/rpc/args_test.go
+++ b/rpc/args_test.go
@@ -2264,7 +2264,7 @@ func TestBlockNumIndexArgsBlocknumInvalid(t *testing.T) {
 }
 
 func TestBlockNumIndexArgsIndexInvalid(t *testing.T) {
-	input := `["0x29a", 1]`
+	input := `["0x29a", true]`
 
 	args := new(BlockNumIndexArgs)
 	str := ExpectInvalidTypeError(json.Unmarshal([]byte(input), &args))