In 1.3.0beta2 both arguments are typehinted.
If the class extending MongoCollection does not do the same you will get the error.
Note: We have introduced several typehints in the 1.3 series, so chances are that several classes will issue similar error messages
This change was required to provide meaningful reflection information, make things more obvious to the user which types are expected - and to be consistent with the documented behaviour.
We are sorry for not having done this from the getgo and needing to break backwards compatibility at this time