a very simple change in the prompt when asking the user to register a new account: tell ahead of time that the password must include a character (besides the mentioned being six char long).

it is frustrating and confusing to have your input rejected and be forced to start afresh for missing a condition which you were not told.

aside: i think asking for a number (or any combination of such things as capitals, etc. etc.) is pretty useless. useful are only longer (non dictionary) passwords - and those should only be asked to protect secrets of value.