git-gui paste not working

I've just upgraded to macOS 10.12.1 and now I can't paste into git-gui 
(the commit message box).

It seems to work if the text is copied from within git-gui but not from 
outside. I've tried using the latest dmg from 
https://git-scm.com/download/mac (2.10.1) but it doesn't fix the problem.