Can't sign my mail

I've just installed Thunderbird Portable, GPG for Thunderbird Portable, and Enigmail. I've created a key pair, and attached it to my account.

However, when I attempt to digitally sign my outgoing mail, I get an error message: Error - bad passphrase. I was never asked to enter in a passphrase, so I can't enter the good one.

Help.

(If you need additional information, just ask.)