"Bad username" error trying to prove Facebook identity

Client: 1.0.18-20170208085140+dc66461
Service: 1.0.18-20170208085140+dc66461
OS: Windows 10 Pro (Version 1607, Build 14393.693)

I'm trying to use the new alpha GUI to prove my Facebook identity. However, no matter why I try, I keep getting a big red "Bad username" error at the top of the window. Unless something's seriously changed that I'm not aware of, Facebook uses e-mail addresses as usernames, but when I enter the e-mail address attached to my FB account, I get this error.

On a hunch, I thought the problem might be the fact that my domain name has a hyphen in it. (I bought this domain way back in 1998, long before I found out hyphens in a domain would be SEO suicide.) This has caused problems for me with several other services in the past that didn't take hyphens into account. Alas, I tried removing the hyphen from the domain and I got the same error. Unfortunately, that probably proves nothing; while I own the same domain both with and without the hyphen, I don't think FB knows about the hyphen-less domain.

If the Keybase GUI doesn't recognize e-mail addresses as FB usernames, what am I supposed to use? If it _does_ use e-mails, why isn't mine being recognized?

No source file or test is named. Start by reproducing the "Bad username" error in the alpha GUI's Facebook identity-proof flow with an email address on Windows 10. Done means establishing whether email addresses are accepted and resolving the reported behavior if they are.

Written by the indexing model from the issue text.