Not sure if it is a bug, but you cannot use a multiselect field as an Urlmap field.
I mean, the system lets you assign it, but it errors every time.
I can sort of see the logic, but there's no reason it couldn't work.
I have a ProgramOfStudy structure that has a 'type' field that allows for UG or GR (or both, hence the multiselect).
So /programs/ug/math and /programs/gr/math should both map to the same contentlet (as it is both ug and gr) but this just fails. Obviously I can work around it, but it was frustrating to figure out why it was not working - I'd like to save someone else that pain - and suggest that either they support it or not allow it to be assigned.
I see that there's an option to use URLMAP_FALLTHROUGH to bypass if someone else stumbles across this message looking for a solution. That would allow me to make my structure work, but at this stage I am just going to split the fields out.