diff --git a/xpcom/glue/nsStringAPI.h b/xpcom/glue/nsStringAPI.h
index 8f88125cfb9..d9202d7bd0b 100644
--- a/xpcom/glue/nsStringAPI.h
+++ b/xpcom/glue/nsStringAPI.h
@@ -1322,7 +1322,8 @@ Substring( const nsAString& str, PRUint32 startPos, PRUint32 length )
 inline const nsDependentSubstring
 Substring( const PRUnichar* start, const PRUnichar* end )
 {
-  return nsDependentSubstring(start, end - start);
+  NS_ABORT_IF_FALSE(PRUint32(end - start) == end - start, "string too long");
+  return nsDependentSubstring(start, PRUint32(end - start));
 }
 
 inline const nsDependentSubstring
@@ -1360,7 +1361,8 @@ inline
 const nsDependentCSubstring
 Substring( const char* start, const char* end )
 {
-  return nsDependentCSubstring(start, end - start);
+  NS_ABORT_IF_FALSE(PRUint32(end - start) == end - start, "string too long");
+  return nsDependentCSubstring(start, PRUint32(end - start));
 }
 
 inline