(More) Questions about stored procedures

I'm still experimenting with stored procedures and I have a couple of
questions that I can't seem to find answers to. Here is the function

CREATE OR REPLACE FUNCTION getdetailamts ( acct_no char(22), beg_moyr
char(6),
end_moyr char(6), OUT beg_bal float8, OUT half_bal float8) AS $$
DECLARE recs record;
BEGIN
beg_bal := 0.0; -- if this is left out the later addition doesn't
occur
half_bal := 0.0;
FOR recs IN SELECT apmoyr, drtype, dramt FROM detrec WHERE
detrec.acctno = $1 AND detrec.apmoyr >= $2 AND detrec.apmoyr <= $3"
LOOP
beg_bal := beg_bal + recs.dramt;
END LOOP;
END; $$ language plpgsql IMMUTABLE STRICT;

My first question is that if I don't initially set beg_bal := 0.0, the
addition in the loop doesn't seem to work. It returns NULL. I don't see
anything in the documentation that explains this behavior. I assume it has
something to do with beg_bal's initial value being NULL?

I change the loop part to:

LOOP
IF recs.apmoyr < $3 THEN beg_bal := beg_bal + recs.dramt;
END LOOP;

Now I get an error when I attempt to create the function. Why?

Thanks, Lynn