google app engine - org.datanucleus.sco.backed.ArrayList cannot be cast to java.util.Set -

since 4 days ago, in random short periods of time, deployed application throwing error:

org.datanucleus.sco.backed.arraylist cannot cast java.util.set

we using gwt 2.4 / java 1.7 (we migrate 1.6 1.7)

it happens when retrieving or persisting entity string set:

import java.util.hashset; import java.util.set; ... @persistencecapable(identitytype = identitytype.application, detachable = "true") public class dbaccount {     @primarykey     @persistent(valuestrategy = idgeneratorstrategy.identity)     private key key; ...     @persistent     private set<string> accounts; ...     public dbaccount(seraccount account) throws exception {         ...         this.accounts= new hashset<string>();         ...         key = keyfactory.createkey(dbcuentas.class.getsimplename(), this.id);     } } 

i have checked entire project reference org.datanucleus.sco.backed.arraylist , not exist.

any idea?

it may sound stupid, have tried use list instead of set ?

after checking out docs again, can't find example google collection other list.

reading issue, looks jpa datanucleus impl indeed using own arraylist implementation whatever do. don't know why problem happens randomly though...