MemCache: java.lang.IllegalArgumentException

Question

I repeatedly get this error while trying to cache my objects in a ArrayList to MemCache in google appengine

java.lang.IllegalArgumentException: Cannot use as value: '[Id: 24, Version: 1, Id: 25, Version: 1]'

The key and value objects are both serializable. I tried to implement Externalizable too but its the same problem. Weirdly one class always throws this exception on the production server but is OK on the dev server. I figured out its because of the cache limit of 1 mb and the cache also considers the properties of the object being saved(I have more data on prod than on dev).

BUT this object I am trying to save is serializable too (including all its properties) and it throws this exception even on the dev server even for a single object in the list.

Any hints?