Problem with FOUND

Hi.
I run a function

CREATE OR REPLACE FUNCTION addRating(tbl_ INTEGER,value_ INTEGER)
RETURNS void AS $$
DECLARE
    tablename TEXT;
    fieldname TEXT;
BEGIN
    tablename:='Rating_'||tbl_;
    fieldname:='val';
    EXECUTE 'UPDATE '||tablename||' SET '||fieldname||'='||value_||'
WHERE '||fieldname||'='||value_ ;
    IF NOT FOUND THEN
        EXECUTE 'INSERT INTO '||tablename||' ('||fieldname||') VALUES ('||value_||')';
    END IF;
END;
$$ LANGUAGE plpgsql;

The UPDATE command works fine (afterwards the table is updated) but it
seems that it do the insert even if it do the update.
I thought this was a correct useage of  "FOUND" so that it either do
the update or the insert, not both.
Can anyone help me spot the error?