--- a/js/src/builtin/TestingFunctions.cpp
+++ b/js/src/builtin/TestingFunctions.cpp
@@ -2373,18 +2373,20 @@ class CloneBufferObject : public NativeO
         obj->discard();
 
         char* str = JS_EncodeString(cx, args[0].toString());
         if (!str)
             return false;
         size_t nbytes = JS_GetStringLength(args[0].toString());
         MOZ_ASSERT(nbytes % sizeof(uint64_t) == 0);
         auto buf = js::MakeUnique<JSStructuredCloneData>(0, 0, nbytes);
-        if (!buf->Init(nbytes, nbytes))
+        if (!buf->Init(nbytes, nbytes)) {
+            JS_free(cx, str);
             return false;
+        }
         js_memcpy(buf->Start(), str, nbytes);
         JS_free(cx, str);
         obj->setData(buf.release());
 
         args.rval().setUndefined();
         return true;
     }