I see this in the log:

$ git log --show-signature 6a34f4ccc8a5d4a48e25ad3c9c512f8634928b91
commit 6a34f4ccc8a5d4a48e25ad3c9c512f8634928b91
gpg: Signature made Wed 28 Dec 2016 08:38:21 PM EST
gpg:                using RSA key 91CCDB9B48541B99
gpg: BAD signature from "Tobias Geerinckx-Rice <me@tobias.gr>" [unknown]
Author: Tobias Geerinckx-Rice <me@tobias.gr>
Date:   Wed Dec 28 23:01:49 2016 +0100

    gnu: python-prompt-toolkit: Update to 1.0.9.
    
    * gnu/packages/python.scm (python-prompt-toolkit): Update to 1.0.9.

Does anyone else get the same result? Any ideas?

Tobias, what version of GnuPG are you using?

I'm using:

gnupg	2.1.17	/gnu/store/1sa7k3x3y3chdl31p7y7x3sa7p9h1qa3-gnupg-2.1.17

... which is built from Guix commit 791ac6a68d1e4bc80f4760cd968cb6c26dbab9ba.