Logged In: YES 
user_id=764

clients.py contains some more highly suspect code around
line 1194:

            if mlaction != 'set':
                raise ValueError, 'You have submitted a
remove action for'\
                    ' the property "%s" which doesn\'t
exist'%propname

this seems to imply that mlaction is either 'set' or
'remove', while
it can be 'add', too ! I may have a form to submit a new
issue where
I allow multiple selections for a multichoice using
checkboxes, using
the ':add:...' technique you introduced recently. This would
fail with the
given code...