release finish -s -u KEY TAG not working in 0.4.1

[jokeyrhyme]

I'm using git flow as installed via homebrew on OS X.

$ git version
git version 2.8.1

$ git flow version
0.4.1

I have a signature here: https://keybase.io/jokeyrhyme

I tried to finish a git flow release with a PGP / GPG signature, but it didn't work:

$ git flow release finish -u keybase.io/jokeyrhyme 2.29.0
gpg: skipped "'keybase.io/jokeyrhyme'": No secret key
gpg: signing failed: No secret key
error: gpg failed to sign the data
error: unable to sign the tag
The tag message has been left in .git/TAG_EDITMSG
Tagging failed. Please run finish again to retry.

I am running gpg-agent properly and everything. I was able to finish the signing by using git directly:

$ git tag -u keybase.io/jokeyrhyme 2.29.0
You need a passphrase to unlock the secret key for
user: "keybase.io/jokeyrhyme <jokeyrhyme@keybase.io>"
2048-bit RSA key, ID 9DD6CC63, created 2014-09-30 (main key ID F96EC3B1)

So my GNUPG2 configuration, keyfile, git and agent all seem to be working as expected. Here's an example of a tag I signed with the same key (verified by GitHub) for a project that does not use git flow:

• https://github.com/jokeyrhyme/appcache-fetcher.js/releases/tag/1.3.2

Given that the -s and -u flags are accepted by git flow release finish ... it seems like this should probably have worked. Am I doing something incorrectly here?