--- a/js/src/jsnum.cpp
+++ b/js/src/jsnum.cpp
@@ -1748,19 +1748,20 @@ js_strtod(ThreadSafeContext *cx, const C
     size_t length = end - s;
 
     Vector<char, 32> chars(cx);
     if (!chars.growByUninitialized(length + 1))
         return false;
 
     size_t i = 0;
     for (; i < length; i++) {
-        if (s[i] >> 8)
+        jschar c = s[i];
+        if (c >> 8)
             break;
-        chars[i] = char(s[i]);
+        chars[i] = char(c);
     }
     chars[i] = 0;
 
     /* Try to parse +Infinity, -Infinity or Infinity. */
     {
         char *afterSign = chars.begin();
         bool negative = (*afterSign == '-');
         if (negative || *afterSign == '+')