class Class[of T]: pass print c.ToString() Error BCE0055: Internal compiler error: Operation is not valid due to the current state of the object.. The attached patch causes GenericConstructedType.Resolve() to check if an entity really should be mapped before trying to pass it to GenericMapping.Map(). But now that I think about it, this issue can occur in other places. So maybe GenericMapping should be responsible for doing that checking so that such checks are localized to one place. GenericMapping.Map() may also be where BOO-973 can be fixed.