diff --git a/stint/io.nim b/stint/io.nim
index a0ea092..884865b 100644
--- a/stint/io.nim
+++ b/stint/io.nim
@@ -213,14 +213,14 @@ func skipPrefixes(current_idx: var int, str: string, radix: range[2..16]) {.inli
     elif str[1] in {'o', 'O'}:
       doAssert radix == 8, "Parsing mismatch, 0o prefix is only valid for an octal number (base 8)"
       current_idx = 2
-    elif str[1] in {'b', 'B'}:      
+    elif str[1] in {'b', 'B'}:
       if radix == 2:
         current_idx = 2
       elif radix == 16:
         # allow something like "0bcdef12345" which is a valid hex
         current_idx = 0
       else:
-        doAssert false, "Parsing mismatch, 0b prefix is only valid for a binary number (base 2), or hex number"      
+        doAssert false, "Parsing mismatch, 0b prefix is only valid for a binary number (base 2), or hex number"
 
 func nextNonBlank(current_idx: var int, s: string) {.inline.} =
   ## Move the current index, skipping white spaces and "_" characters.
@@ -333,7 +333,7 @@ func toString*[bits: static[int]](num: StUint[bits], radix: static[uint8] = 10):
 
   reverse(result)
 
-func toString*[bits: static[int]](num: StInt[bits], radix: static[int8] = 10): string =
+func toString*[bits: static[int]](num: StInt[bits], radix: static[uint8] = 10): string =
   ## Convert a Stint or StUint to string.
   ## In case of negative numbers:
   ##   - they are prefixed with "-" for base 10.
@@ -466,26 +466,16 @@ template toBytesBE*[bits: static[int]](n: StInt[bits]): array[bits div 8, byte]
 
 {.pop.}
 
-func getRadix(s: static string): uint8 {.compileTime.} =
-  if s.len <= 2:
-    return 10
-  
-  # maybe have prefix have prefix
-  if s[0] != '0':
-    return 10
+include
+  ./private/custom_literal
 
-  if s[1] == 'b':
-    return 2
-      
-  if s[1] == 'o':
-    return 8
-    
-  if s[1] == 'x':
-    return 16
-  
 func customLiteral*(T: type SomeBigInteger, s: static string): T =
   when s.len == 0:
     doAssert(false, "customLiteral cannot accept param with zero length")
-    
+
   const radix = getRadix(s)
+  type TT = T
+  when isOverflow(TT, s, radix):
+    {.error: "Stint custom literal overlow detected" .}
+
   parse(s, T, radix)
diff --git a/stint/private/custom_literal.nim b/stint/private/custom_literal.nim
new file mode 100644
index 0000000..cbba852
--- /dev/null
+++ b/stint/private/custom_literal.nim
@@ -0,0 +1,47 @@
+# Stint
+# Copyright 2018-2023 Status Research & Development GmbH
+# Licensed under either of
+#
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+#
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+func getRadix(s: static string): uint8 {.compileTime.} =
+  if s.len <= 2:
+    return 10
+
+  # maybe have prefix have prefix
+  if s[0] != '0':
+    return 10
+
+  if s[1] == 'b':
+    return 2
+
+  if s[1] == 'o':
+    return 8
+
+  if s[1] == 'x':
+    return 16
+
+func stripPrefix(s: string): string {.compileTime.} =
+  if s[0] != '0':
+    return s
+  if s[1] in {'b', 'o', 'x'}:
+    return s[2 .. ^1]
+  s
+
+func stripLeadingZeros(value: string): string {.compileTime.} =
+  var cidx = 0
+  # ignore the last character so we retain '0' on zero value
+  while cidx < value.len - 1 and value[cidx] == '0':
+    cidx.inc
+  value[cidx .. ^1]
+
+func isOverflow(T: type SomeBigInteger, s: static string, radix: static uint8): bool {.compileTime.} =
+  # a stupid but effective overflow detection
+  # it's a compiletime check anyway
+  let tmp = parse(s, T, radix)
+  let litStr = tmp.toString(radix)
+  let normalizedSrc = s.stripPrefix.stripLeadingZeros
+  litStr != normalizedSrc
diff --git a/tests/test_features.nim b/tests/test_features.nim
index 814e87f..7534905 100644
--- a/tests/test_features.nim
+++ b/tests/test_features.nim
@@ -6,7 +6,12 @@
 #
 # at your option. This file may not be copied, modified, or distributed except according to those terms.
 
-import ../stint, unittest
+import
+  ../stint,
+  unittest
+
+template reject(code: untyped) =
+  static: assert(not compiles(code))
 
 suite "new features":
   test "custom literal":
@@ -19,6 +24,8 @@ suite "new features":
     let
       x = 0b111100011'u128
       y = 0o777766666'u256
+      z = 0x1122334455667788991011121314151617181920aabbccddeeffb1b2b3b4b500'u256
+      w = 340282366920938463463374607431768211455'u128
 
     check:
       a == 0xabcdef0123456.u128
@@ -27,3 +34,21 @@ suite "new features":
       d == -50000.i256
       x == 0b111100011.u128
       y == 0o777766666.u256
+      z == UInt256.fromHex("0x1122334455667788991011121314151617181920aabbccddeeffb1b2b3b4b500")
+      w == UInt128.fromDecimal("340282366920938463463374607431768211455")
+
+  test "custom literal overflow":
+    reject:
+      const
+        z = 0x1122334455667788991011121314151617181920aabbccddeeffb1b2b3b4b5700'u256
+      doAssert(false)
+
+    reject:
+      let
+        z = 0x1122334455667788991011121314151617181920aabbccddeeffb1b2b3b4b5700'u256
+      doAssert(false)
+
+    reject:
+      const
+        w = 1122334455667788991011121314151617181920'u128
+      doAssert(false)