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 ChristopherA@LifeWithAlacrity.com` 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
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](https://github.com/ChristopherA/dotfiles) using [my GPG key](https://github.com/christophera.gpg) and it shows as [verified](https://github.com/ChristopherA/dotfiles/commit/4b1bc47eb4b9177cc6c08448d2d62960b75628a7).

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

Ideas?