diff --git a/string/public/nsReadableUtils.h b/string/public/nsReadableUtils.h
index 6431fe5e98b..ebc59cf9011 100755
--- a/string/public/nsReadableUtils.h
+++ b/string/public/nsReadableUtils.h
@@ -133,9 +133,10 @@ ToUpperCase(basic_nsAWritableString<CharT>& aString)
 
     while ( start != end ) {
       PRUint32 fraglen = start.size_forward();
+      PRUint32 num_bytes = fraglen;
       CharT *buf = start.get();
 
-      while ( fraglen-- ) {
+      while ( num_bytes-- ) {
         *buf = nsCRT::ToUpper(*buf);
         ++buf;
       }
@@ -156,9 +157,10 @@ ToLowerCase(basic_nsAWritableString<CharT>& aString)
 
     while ( start != end ) {
       PRUint32 fraglen = start.size_forward();
+      PRUint32 num_bytes = fraglen;
       CharT *buf = start.get();
 
-      while ( fraglen-- ) {
+      while ( num_bytes-- ) {
         *buf = nsCRT::ToLower(*buf);
         ++buf;
       }
diff --git a/xpcom/ds/nsReadableUtils.h b/xpcom/ds/nsReadableUtils.h
index 6431fe5e98b..ebc59cf9011 100755
--- a/xpcom/ds/nsReadableUtils.h
+++ b/xpcom/ds/nsReadableUtils.h
@@ -133,9 +133,10 @@ ToUpperCase(basic_nsAWritableString<CharT>& aString)
 
     while ( start != end ) {
       PRUint32 fraglen = start.size_forward();
+      PRUint32 num_bytes = fraglen;
       CharT *buf = start.get();
 
-      while ( fraglen-- ) {
+      while ( num_bytes-- ) {
         *buf = nsCRT::ToUpper(*buf);
         ++buf;
       }
@@ -156,9 +157,10 @@ ToLowerCase(basic_nsAWritableString<CharT>& aString)
 
     while ( start != end ) {
       PRUint32 fraglen = start.size_forward();
+      PRUint32 num_bytes = fraglen;
       CharT *buf = start.get();
 
-      while ( fraglen-- ) {
+      while ( num_bytes-- ) {
         *buf = nsCRT::ToLower(*buf);
         ++buf;
       }
diff --git a/xpcom/string/public/nsReadableUtils.h b/xpcom/string/public/nsReadableUtils.h
index 6431fe5e98b..ebc59cf9011 100755
--- a/xpcom/string/public/nsReadableUtils.h
+++ b/xpcom/string/public/nsReadableUtils.h
@@ -133,9 +133,10 @@ ToUpperCase(basic_nsAWritableString<CharT>& aString)
 
     while ( start != end ) {
       PRUint32 fraglen = start.size_forward();
+      PRUint32 num_bytes = fraglen;
       CharT *buf = start.get();
 
-      while ( fraglen-- ) {
+      while ( num_bytes-- ) {
         *buf = nsCRT::ToUpper(*buf);
         ++buf;
       }
@@ -156,9 +157,10 @@ ToLowerCase(basic_nsAWritableString<CharT>& aString)
 
     while ( start != end ) {
       PRUint32 fraglen = start.size_forward();
+      PRUint32 num_bytes = fraglen;
       CharT *buf = start.get();
 
-      while ( fraglen-- ) {
+      while ( num_bytes-- ) {
         *buf = nsCRT::ToLower(*buf);
         ++buf;
       }