null username being set from register

(I suspect I've mucked things up along the way, now when I submit a user through my customized register.html, I get 

> `IntegrityError: (psycopg2.errors.NotNullViolation) null value in column "username" of relation "users" violates not-null constraint`

All the other fields are being parsed properly.

Is that an obvious clue to what I've broken?
