Not able to adjust kerning

Every now and then I’m running into a kerning-pair that I cannot adjust. I keep changing the value, but the value stays the same. Usually it helps to restart Glyphs. After that it’s fine.
Now I would like to make a kerning-exception between Q and period but I can’t change the value. Restarting Glyphs or downloading the latest beta didn’t help.

You are removing the value. that is equivalent of removing the exception. You need to put in a value, in your case that might be 0 (zero).

Ah, damnit!
Thanks Georg!