Non consistent response when setting the keyspace in query explicitly

Using the python driver this works and the query is prepared against my_keyspace, system is ignored.

prepared_statement = self.session.prepare("SELECT * from my_keyspace.my_table", keyspace="system")

However this throws a InvalidRequest: Error from server: code=2200 [Invalid query] message="keyspace madeup_ks does not exist"

prepared_statement = self.session.prepare("SELECT * from my_keyspace.my_table", keyspace="madeup_ks")