What does PuLP LpStatus=Undefined actually mean?

When I add a particular constraint to my problem, the LpStatus of the problem after solving changes to "Undefined" (without this constraint, it was "Optimal"). At the top of this page, the possibilities of the return status are shown, but it doesn't seem to explain what they mean. Can anyone explain what an "undefined" status means? It it something like a syntax error in specifying the constraint?