error in LINQ query "Object reference not set to an instance of an object"

 Contact party =
                       (from c in myContext.cav_configurationSet
                        join p in myContext.ContactSet.DefaultIfEmpty()
                        on c.mm_name equals p.FullName
                        where c.mm_name == "TransactionPerformedBy" &&
                        c.StateCode == cav_configurationState.Active
                        select p).FirstOrDefault().ToEntity<Contact>();