not null error in trigger on unrelated column

I have a members table and an items table. Every time items.member_id or items.active gets
changed, members.items_submitted and members.items_approved gets updated by a trigger on items. I
added an "admin" column to members, and now this happens:

=> update items set active = false where member_id=38;
ERROR: null value in column "admin" violates not-null constraint
CONTEXT: SQL statement "update members set items_approved=items_approved-1 where id= $1 "
PL/pgSQL function "update_member_item_counts" line 54 at SQL statement

The relevant part of the (after) trigger function on items is:

ELSIF OLD.active is true and NEW.active is false then
update members set
items_approved=items_approved-1
where id=NEW.member_id;
END IF;

Is it necessary to drop and recreate triggers and/or corresponding functions after changing a
table's schema? I don't know how something is trying to set members.admin to null (table members
has no triggers).

thanks
csn
Postgresql 8.0.x

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