diff --git a/mozilla/js/src/jsinterp.c b/mozilla/js/src/jsinterp.c
index 5e3608eb650..ed9dbcc0f19 100644
--- a/mozilla/js/src/jsinterp.c
+++ b/mozilla/js/src/jsinterp.c
@@ -4389,13 +4389,14 @@ interrupt:
                 str = JSVAL_TO_STRING(lval);
                 i = JSVAL_TO_INT(rval);
                 if ((size_t)i < JSSTRING_LENGTH(str)) {
-                    str = js_GetUnitString(cx, JSSTRING_CHARS(str)[i]);
+                    str = js_GetUnitString(cx, str, (size_t)i);
                     if (!str)
                         goto error;
                     rval = STRING_TO_JSVAL(str);
                     goto end_getelem;
                 }
             }
+
             VALUE_TO_OBJECT(cx, -2, lval, obj);
             if (JSVAL_IS_INT(rval)) {
                 if (OBJ_IS_DENSE_ARRAY(cx, obj)) {
@@ -4415,6 +4416,7 @@ interrupt:
                 if (!InternNonIntElementId(cx, obj, rval, &id))
                     goto error;
             }
+
             if (!OBJ_GET_PROPERTY(cx, obj, id, &rval))
                 goto error;
           end_getelem:
diff --git a/mozilla/js/src/jsstr.c b/mozilla/js/src/jsstr.c
index 0d9bde4371d..41a8eacb47b 100644
--- a/mozilla/js/src/jsstr.c
+++ b/mozilla/js/src/jsstr.c
@@ -583,7 +583,7 @@ str_resolve(JSContext *cx, JSObject *obj, jsval id, uintN flags,
 
     slot = JSVAL_TO_INT(id);
     if ((size_t)slot < JSSTRING_LENGTH(str)) {
-        str1 = js_GetUnitString(cx, JSSTRING_CHARS(str)[slot]);
+        str1 = js_GetUnitString(cx, str, (size_t)slot);
         if (!str1)
             return JS_FALSE;
         if (!OBJ_DEFINE_PROPERTY(cx, obj, INT_TO_JSID(slot),
@@ -849,7 +849,6 @@ str_charAt(JSContext *cx, uintN argc, jsval *vp)
     jsval t, v;
     JSString *str;
     jsint i;
-    jschar c;
     jsdouble d;
 
     t = vp[1];
@@ -877,10 +876,7 @@ str_charAt(JSContext *cx, uintN argc, jsval *vp)
         i = (jsint) d;
     }
 
-    c = JSSTRING_CHARS(str)[i];
-    str = (c < UNIT_STRING_LIMIT)
-          ? js_GetUnitString(cx, c)
-          : js_NewDependentString(cx, str, i, 1);
+    str = js_GetUnitString(cx, str, (size_t)i);
     if (!str)
         return JS_FALSE;
     *vp = STRING_TO_JSVAL(str);
@@ -1961,7 +1957,6 @@ str_slice(JSContext *cx, uintN argc, jsval *vp)
 {
     jsval t, v;
     JSString *str;
-    jschar c;
 
     t = vp[1];
     v = vp[2];
@@ -1976,9 +1971,8 @@ str_slice(JSContext *cx, uintN argc, jsval *vp)
             if (length == 0) {
                 str = cx->runtime->emptyString;
             } else {
-                str = (length == 1 &&
-                       (c = JSSTRING_CHARS(str)[begin]) < UNIT_STRING_LIMIT)
-                      ? js_GetUnitString(cx, c)
+                str = (length == 1)
+                      ? js_GetUnitString(cx, str, begin)
                       : js_NewDependentString(cx, str, begin, length);
                 if (!str)
                     return JS_FALSE;
@@ -2353,13 +2347,17 @@ js_InitDeflatedStringCache(JSRuntime *rt)
     IN_UNIT_STRING_SPACE((rt)->unitStrings, cp)
 
 JSString *
-js_GetUnitString(JSContext *cx, jschar c)
+js_GetUnitString(JSContext *cx, JSString *str, size_t index)
 {
+    jschar c, *cp, i;
     JSRuntime *rt;
-    JSString **sp, *str;
-    jschar *cp, i;
+    JSString **sp;
+
+    JS_ASSERT(index < JSSTRING_LENGTH(str));
+    c = JSSTRING_CHARS(str)[index];
+    if (c >= UNIT_STRING_LIMIT)
+        return js_NewDependentString(cx, str, index, 1);
 
-    JS_ASSERT(c < UNIT_STRING_LIMIT);
     rt = cx->runtime;
     if (!rt->unitStrings) {
         sp = (JSString **) calloc(UNIT_STRING_LIMIT * sizeof(JSString *) +
diff --git a/mozilla/js/src/jsstr.h b/mozilla/js/src/jsstr.h
index dca86d1bfd3..1add234d3da 100644
--- a/mozilla/js/src/jsstr.h
+++ b/mozilla/js/src/jsstr.h
@@ -407,14 +407,11 @@ js_InitDeflatedStringCache(JSRuntime *rt);
 #define UNIT_STRING_LIMIT 256U
 
 /*
- * Get the independent string containing only character code c (backstopped
- * with a NUL as usual for independent strings).
- *
- * This function must be called only for c < UNIT_STRING_LIMIT. It asserts to
- * insist on this requirement in DEBUG builds.
+ * Get the independent string containing only character code at index in str
+ * (backstopped with a zero character as usual for independent strings).
  */
 extern JSString *
-js_GetUnitString(JSContext *cx, jschar c);
+js_GetUnitString(JSContext *cx, JSString *str, size_t index);
 
 extern void
 js_FinishUnitStrings(JSRuntime *rt);