Inherent insert using WHERE partitioning is incomplete

Using a metadata structure such as:

create foreign table b (custid integer, field1 varchar) options updatable true)
create view c options (updatable true) as select * from b where custid = 1 union all select * from b where custid = 2;

will fail to process INSERT into c (custid, field1) values (1, 'foo')

because a null value will be inserted into be for the custid field. The logic is treating the WHERE partitioning as if it were projected literals from the SELECT cause inappropriately.