[BUG] Failed to sign commit

Recently I've updated a bunch of stuff, including git and gpg. I'm using

- mac OS 10.10.5
- git 2.13.1
- gpg (GnuPG) 2.1.21 / libgcrypt 1.7.7

When I do

$ git commit --allow-empty -v -m "lol" error: gpg failed to sign the data fatal: failed to write commit object

I tried the verbose flag hoping to have a better insight, but not very useful. Not sure if it's a gpg problem, a git problem, or something else.

Any clue on how to debug the problem? Do you need any gpg output to better understand the problem?

Thanks, Pedro