Problem creating trigger

Using PostgreSQL 9.1 I have created my first function. Heres the create statement reported by pgAdminIII:

CREATE OR REPLACE FUNCTION detect_branch(integer)
RETURNS integer AS
' UPDATE company SET co_has_branch_sa=CASE WHEN (SELECT sum(br_ca_rate_sa) FROM branch WHERE br_co_id=$1)>0 THEN TRUE ELSE FALSE END
WHERE co_id=$1 RETURNING 1; '
LANGUAGE sql VOLATILE
COST 100;
ALTER FUNCTION detect_branch(integer)
OWNER TO postgres;

Works well.

However, if I try to create a trigger:
CREATE TRIGGER run_detect_branch
AFTER UPDATE OR INSERT ON branch
FOR EACH ROW
EXECUTE PROCEDURE detect_branch()

I get an error "function detect_branch() does not exist".

Can some kind soul tell me why?

Michael Rowan
mike(dot)rowan(at)internode(dot)on(dot)net

11 Kingscote Street
ALBERTON
South Australia 5014

tel 618 8240 3993
mob 0417 812 509