Function Parameters in GROUP BY clause cause errors

When I use a parameter in a query which aggregates it fails with a GROUP BY error. What syntax can I use to avoid this error?

CREATE TABLE test (email_creation_datetime timestamp);
INSERT INTO test VALUES ('2006-03-20 09:00');
INSERT INTO test VALUES ('2006-03-20 09:15');
INSERT INTO test VALUES ('2006-03-20 09:30');
INSERT INTO test VALUES ('2006-03-20 09:45');

Query without parameters works fine:

select to_char(to_timestamp(EXTRACT(HOUR FROM em.email_creation_datetime) || ':' || (EXTRACT(MINUTE FROM em.email_creation_datetime)::integer/30) * 30, 'HH24:MI'), 'HH24:MI') as TheInterval
from test em
group by to_char(to_timestamp(EXTRACT(HOUR FROM em.email_creation_datetime) || ':' || (EXTRACT(MINUTE FROM em.email_creation_datetime)::integer/30) * 30, 'HH24:MI') , 'HH24:MI')

theinterval
09:30
09:00

But the same query with a parameter returns a GROUP BY error:

CREATE OR REPLACE FUNCTION EmailByInterval(IntervalMinutes int) RETURNS SETOF test AS $$
DECLARE rec RECORD;
BEGIN
FOR rec IN
select to_char(to_timestamp(EXTRACT(HOUR FROM em.email_creation_datetime) || ':' || (EXTRACT(MINUTE FROM em.email_creation_datetime)::integer/IntervalMinutes) * IntervalMinutes, 'HH24:MI'), 'HH24:MI') as TheInterval
from test em
group by to_char(to_timestamp(EXTRACT(HOUR FROM em.email_creation_datetime) || ':' || (EXTRACT(MINUTE FROM em.email_creation_datetime)::integer/IntervalMinutes) * IntervalMinutes, 'HH24:MI') , 'HH24:MI')
LOOP
RETURN NEXT rec;
END LOOP;
RETURN;
END;
$$ LANGUAGE plpgsql;

Query returned successfully with no result in 70 ms.

select * from emailbyinterval(30);

ERROR: column "em.email_creation_datetime" must appear in the GROUP BY clause or be used in an aggregate function
CONTEXT: SQL statement " select to_char(to_timestamp(EXTRACT(HOUR FROM em.email_creation_datetime) || ':' || (EXTRACT(MINUTE FROM em.email_creation_datetime)::integer/ $1 ) * $2 , 'HH24:MI'), 'HH24:MI') as TheInterval from test em group by to_char(to_timestamp(EXTRACT(HOUR FROM em.email_creation_datetime) || ':' || (EXTRACT(MINUTE FROM em.email_creation_datetime)::integer/ $3 ) * $4 , 'HH24:MI') , 'HH24:MI')"
PL/pgSQL function "emailbyinterval" line 3 at for over select rows