Error message: Sprint must exist: -1

Steps to Reproduce:

1. Have a JIRA Agile Scrum project created
2. Have issue(s) with a completed sprint
   
3. Try to edit the issue
   

Expected Results:

1. Users should be able to update the issue without having to fill-out the sprint field.

Actual Results:

1. The following error message shows up and users are not able to edit/update the issue.

   Sprint must exist: -1

   

Add-on Version:
JIRA Agile v6.7.1-D20
JIRA v6.5-OD-03

Notes / Workaround:
This error doesn't occur if an active sprint is added to the Sprint field when the issue is edited.
Update: This error is occurring even if you select an active sprint.