500 error when dealing with Issue labels

When I try to create a new Issue with a label (the auto-generated labels), or if I try to edit an issue that has no label and try to add a label, I get an error 500.

GitLab 7.7.2

GitLab Shell 2.4.2

GitLab API v3

Ruby 2.2.0p0

Rails 4.1.1