Message verification on /verify not working

I tried to verify a message at keybase.io/verify but when I hit the Verify button, I get "Error: no header found." There's no indication on the page what header I need to add.

Start by reproducing the failure at keybase.io/verify with the message used for verification and inspect what happens when Verify is pressed. Trace the page's verification flow to determine why it reports "Error: no header found"; done means the expected verification behavior is clear and the error is resolved or explained on the page.

Written by the indexing model from the issue text.