Seen on JIRA versions 5.0.6 and 5.1.1. Not seen on JIRA 5.2.

Steps:

1. Start JIRA instance with GH 6.0.9-SNAPSHOT
2. Create an epic
3. Restore JIRA data from pre-Epics (i.e. data which does not have Epic fields already created)

I will have to do some more investigation to see whether its JIRA version specific.

Note: did not see the same exception in the logs as GHS-6710