Error: "Unusable Public Key"

I am using macOS Catalina and the brew version of git-crypt (there is no -v option so I am not sure which version of git-crypt, but it is the todays brew install git-crypt).

I have a repo without git-crypt (my .dotfiles) and I have just initialized it with git-crypt init and git-crypt answers Generating key...

However, when I git-crypt add-gpg-user [email protected] I get:

gpg: 0xFDFE14A54ECB30FC5D2274EFF8D36C91357405ED: skipped: Unusable public key
gpg: [stdin]: encryption failed: Unusable public key
git-crypt: GPG error: Failed to encrypt

git-crypt is finding my key (thus the fingerprint in response), but says it is "unusable".

gpg --list-keys looks good as well:

# gpg --list-keys                                  (master) [~/.dotfiles]
/Users/christophera/.gnupg/pubring.kbx
--------------------------------------
pub   rsa4096 2015-04-16 [SC] [expires: 2020-04-16]
      FDFE14A54ECB30FC5D2274EFF8D36C91357405ED
uid           [ unknown] Christopher Allen <[email protected]>
uid           [ unknown] [jpeg image of size 9272]

A few points: that GPG key is properly signed --lsign-key, and has ultimate trust "5", and I am able to use that commit to my dotfiles repo using my GPG key and it shows as verified.

So in all other ways my GPG key works. But I can't add myself as the first git-crypt user.

Ideas?