diff --git a/dom/src/base/nsJSEnvironment.cpp b/dom/src/base/nsJSEnvironment.cpp
index 501d3b6c0791..f03cba03940c 100644
--- a/dom/src/base/nsJSEnvironment.cpp
+++ b/dom/src/base/nsJSEnvironment.cpp
@@ -20,9 +20,6 @@
  * Contributor(s): 
  */
 
-#ifdef NS_DEBUG
-#include <ctype.h>
-#endif
 #include "nsJSEnvironment.h"
 #include "nsIScriptObjectOwner.h"
 #include "nsIScriptContextOwner.h"
@@ -460,7 +457,7 @@ AtomToEventHandlerName(nsIAtom *aName, char *charName, PRUint32 charNameSize)
     c = char(name[i]);
 
     // The HTML content sink must have folded to lowercase already.
-    NS_ASSERTION(c == '\0' || islower(c), "non-alphabetic event handler name");
+    NS_ASSERTION(c == '\0' || ('a' <= c && c <= 'z'), "non-alphabetic event handler name");
 
     NS_ASSERTION(i < charNameSize, "overlong event handler name");
     charName[i++] = c;