Hi Mary,
I am completely agree with Maciej, that if you didn't use a fork then you didn't need the join. I can understand your feeling because there are two incoming transitions so you want to tell people that both of them should do same thing after the JoinActivity.
This exception might be cause be an already known issue https://jira.jboss.org/jira/browse/JBPM-2607, which has been fixed and will be released in jBPM-4.4.