replace null with 0 in subselect ?

Hello,
I have a statement like this :

INSERT INTO tab1 (c1, c2, c3) VALUES (1,2, SELECT MAX(pos)+1 FROM tab2);

This works fine if the subselect returns a value, but if it returns
null there is a problem. In this case a 0 has to be inserted.
Is there any pg function which solves this problem ?
I know that oracle has a function but I didn't find
something similar in pg.

Thanks
berger