Problem casting my Session to SessionImpl

[Kurz Wolfgang]

Hello everyone,

i am trying to implement the garbage collection for the datastore, which looks 
really easy in the wiki.

Here is the code I use:

ObjectContentManager ocm = ocmFactory.createObjectContentManager();
GarbageCollector gc;
SessionImpl si = (SessionImpl)ocm.getSession();
gc = si.createDataStoreGarbageCollector();      
gc.scan();
gc.stopScan();

but then I get the following exception:

javax.el.ELException: java.lang.ClassCastException: 
org.apache.jackrabbit.core.XASessionImpl cannot be cast to 
org.apache.jackrabbit.core.SessionImpl

I am using a local Repository which is globally available over JNDI.

Anyone have an Idea how I get the this working?

Thx a lot in advance to anyone who has some hints for me:-)

Wolfgang