another trigger problem

I have another problem with a slightly different trigger. It's very weird,
because it is exactly the same as the first trigger, that now works, except
for the table name.

The error is:

ERROR: query string argument of EXECUTE is null
CONTEXT: PL/pgSQL function metric_int_insert_func() line 5 at EXECUTE
statement

The trigger is:

CREATE OR REPLACE FUNCTION metric_int_insert_func()
RETURNS TRIGGER AS $$
DECLARE insert_sql text;
BEGIN
insert_sql:='insert into metric_int_values_' ||
to_char(NEW.datetimeval,'YYYYMM') || ' values ($1.*)';
EXECUTE insert_sql using NEW;
RETURN NULL;
END;
$$
LANGUAGE plpgsql;

DROP TRIGGER IF EXISTS insert_metric_int_insert_trigger on
metric_int_values;
CREATE TRIGGER insert_metric_int_insert_trigger
BEFORE INSERT ON metric_int_values
FOR EACH ROW EXECUTE PROCEDURE metric_int_insert_func();

which is exactly the same as this one that works:
CREATE OR REPLACE FUNCTION metric_double_insert_func()
RETURNS TRIGGER AS $$
DECLARE insert_sql text;
BEGIN
insert_sql:='insert into metric_double_values_' ||
to_char(NEW.datetimeval,'YYYYMM') || ' values ($1.*)';
EXECUTE insert_sql using NEW;
RETURN NULL;
END;
$$
LANGUAGE plpgsql;

DROP TRIGGER IF EXISTS insert_metric_double_insert_trigger on
metric_double_values;
CREATE TRIGGER insert_metric_double_insert_trigger
BEFORE INSERT ON metric_double_values
FOR EACH ROW EXECUTE PROCEDURE metric_double_insert_func();

I can't seem to figure it out. I've retyped some of the lines, in case
there is a weird character somewhere, but they got there with a vi yank and
put, so that's not likely.

Anyone have any ideas?

Thanks,
Susan