I got an issue stuck in a state where it was "In Progress" and not assigned to anyone and any attempts to assign it resulted in a system error.

Steps to reproduce:
1. Create an issue
2. Assign it to yourself
3. Mark it is "In Progress"
4. Edit the issue changing the issue to be unassigned in the edit form
5. Assign it to someone

Expected results:
After step 4 the issue should be unassigned and marked as "Open".
After step 5 the issue thould be "Open" and assigned to the selected user

Actual results:
After step 4 the issue is unassigned but still marked as "In Progress"
Step 5 generates a System Error and fails
To recover I had to "Resolve" and "Reopen" the issue so it could be assigned again.