Custom Externalizers cause NotSerializableException

Caused by: org.infinispan.marshall.NotSerializableException: net.beaumaris.odrm.resmanagement.type.ODRMSessionData$ODRMSessionDataExternalizer
Caused by: an exception which occurred:
        in field externalizer
        in object org.infinispan.marshall.jboss.JBossExternalizerAdapter@2c370555
                -> toString = org.infinispan.marshall.jboss.JBossExternalizerAdapter@2c370555
        in object net.beaumaris.odrm.resmanagement.type.ODRMSessionData@62c91115
                -> toString = ODRMSessionData [sopId=999991, segmentIndex=0, streamerId=99999]
        in object org.infinispan.commands.write.PutKeyValueCommand@cf4d21f7
                -> toString = PutKeyValueCommand{key=9d3d0a41-4d2a-4320-aa38-64a5203c49bf, value=ODRMSessionData [sopId=999991, segmentIndex=0, streamerId=99999], flags=[SKIP_REMOTE_LOOKUP], putIfAbsent=false, lifespanMillis=-1, maxIdleTimeMillis=-1}
        in object org.infinispan.commands.remote.SingleRpcCommand@280f4ad0
                -> toString = SingleRpcCommand{cacheName='ODRM_SESSION', command=PutKeyValueCommand{key=9d3d0a41-4d2a-4320-aa38-64a5203c49bf, value=ODRMSessionData [sopId=999991, segmentIndex=0, streamerId=99999], flags=[SKIP_REMOTE_LOOKUP], putIfAbsent=false, lifespanMillis=-1, maxIdleTimeMillis=-1}}