Priority with negative order is incorrectly handled

I'm not sure, if it is SHOULD be possible for prioritie's order to have negative values (less than 0).
It certainly possible in UI.
But bad things happens, when I did that:

• The negative priority doesn't show up in the list of priorities (in the SETTINGS dialog). So, it's impossible to fix the order here. You can fix it in Gitlab itself.
• Labels are shown instead of priorities. See screenshot.

I think either negative values shall be handled/parsed correctly OR they shall not be allowed

Issue found in version 1.6.1