[Dovecot] passwd-file - lower limit for username?

Hi, previously I could use the following in my passwd-file:-

AB:{plain}password

I'm not sure when it stopped working, its not an account I access with any regularity, but today it would not work (2.1.0, 64-bit Arch Linux), with an 'unknown user' error in the logs. Is there some new lower limit for username length?