--- a/js/src/jsstr.cpp
+++ b/js/src/jsstr.cpp
@@ -207,17 +207,17 @@ RopeAllocSize(const size_t length, size_
     /*
      * Grow by 12.5% if the buffer is very large. Otherwise, round up to the
      * next power of 2. This is similar to what we do with arrays; see
      * JSObject::ensureDenseArrayElements.
      */
     if (length > ROPE_DOUBLING_MAX)
         size = minCap + (minCap / 8);
     else
-        size = 1 << (JS_CeilingLog2(minCap));
+        size = RoundUpPow2(minCap);
     *capacity = (size / sizeof(jschar)) - 1;
     JS_ASSERT(size >= sizeof(JSRopeBufferInfo));
     return size;
 }
 
 static JS_ALWAYS_INLINE JSRopeBufferInfo *
 ObtainRopeBuffer(JSContext *cx, bool usingLeft, bool usingRight,
                  JSRopeBufferInfo *sourceBuffer, size_t length,