Enigmail: Key not found or not valid? Install pinentry

I recently upgraded PC-BSD from scratch, and was dismayed to find that Enigmail no longer worked correctly and provided me with a rather misleading dialog box:

Send operation aborted. Key 0xABCDEF01 not found or not valid. The (sub-)key might have expired.

After bashing my head against the wall for a little while, I finally had the bright idea of trying to encrypt a message from the command line:

baitisj@foo:/tmp$ gpg -e -r adele@gnupp.de -a -s

gpg-agent[10380]: can't connect to the PIN entry module: IPC connect call failed
gpg-agent[10380]: command get_passphrase failed: No pinentry

...

baitisj@foo:/tmp$ sudo pkg install pinentry-gtk2

Victory!