Membership is allowing spaces during signup. Screwing up BuddyPress

Hi, when a new user registers, they are asked for a user name and allowed to type in something like "Jess Finn" (with the space).

Then, when they go to their BuddyPress profile (via the "Sidebar Login" plugin which has %username% in a url to get to profile), they're getting "Page Not Found" errors.

How can I fix it so that users aren't allowed to use a user name with a space?

Thanks!