Stopping expansion of periods in mbox name?

[Ben <bender647@xxxxxxxxx>]

I recently switched mutt versions on one of my computers to 1.5.19
from the 1.4 series.  A problem I'm having is that mutt is expanding
periods in mbox names to forward slashes, as if talking to a Maildir
imap server.

For example, I request a change-folder to imap://server/firstname.lastname
and I get error Can't open mailbox mail/firstname/lastname.

I've set mbox_type=mbox

Using mutt 1.4.2.3i with the same configs doesn't do this.

Any ideas?