SQL delete not in related table

["Jeff Crosby" <jlcrosby@xxxxxxxxxxxxxxxx>]

OK, I've spent about an hour trying to figure this out.  SQL does not like
the following statement:

DELETE FROM PODTL WHERE (VNDNR, WEEKX, DIGIT) NOT IN               
                        (SELECT (VNDNR, WEEKX, DIGIT) FROM POHDR);

I did this identical thing a half dozen times yesterday *EXCEPT* there was
only one field name to match on, not 3.  I've looked at manuals and googled.
I've tried with/without parens, with/without commas to no avail.  All the
examples I find do it on only one field.

What's wrong with the syntax?