Just in case anyone is still looking for an answer to this issue, I just ran into the same problem. During my investigation, I learned that BlazeDS serialization is expecting only String key values for Map objects. Since the Map coming in has Integer keys, the key is changed to a string by the serialization. At that point, the value cannot be obtained and serialized. Change the Integer key to a String and it works. Here is the site that helped me get it working.