Imp issuing improper "SORT" command at login?

I have a current up-to-date-via-pear install of horde/imp webmail, using 
imp's IMAP for user authentication.
This setup was working well until recently.

Horde/imp is now reporting "IMAP error reported by server." at user login.
Capturing the IMAP conversation shows that horde/imp is issuing an 
improper IMAP "SORT" command, leaving out the mandatory charset 
specification, at login.  This breaks imp functionality, and appears to 
be a reasonably recent change in behavior.

The IMAP command being issued by horde/imp is:
"<n> UID SORT (ARRIVAL) ALL"

when the proper structure of this command is something like:
"<n> UID SORT (ARRIVAL) UTF-8 ALL"
or
"<n> UID SORT (ARRIVAL) US-ASCII ALL"

Is this a configuration mistake on my part, or is this a bug?
Do I need to explicitly set/declare a charset somewhere in my configuration?
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