Devise mass assignment error for failed logins when using username as login

HAving a weird one.

I've set up devise to authenticate using either a username or an email as per Devise login with username

I can log in absolutely fine with a correct username or email with a password. If I put something incorrect in, I get a

Can't mass-assign protected attributes: login

Can't work out quite what's going on. I assume it's something to do with Devise killing the session, but not sure why it would be doing anything with the login field at that point....