Problem with trigger function

I am trying to implement a trigger in a PostgreSQL-9.6.17 database:

CREATE OR REPLACE FUNCTION customer_num_informix()
RETURNS trigger AS $$
BEGIN
-- An Aubit/Informix 4GL INSERT statement passes the value 0 to Informix
-- DBMS for columns that have the SERIAL data type. Informix will then
-- use the incremented serial number in place of 0. PostgreSQL instead
-- will simply take the value 0 and replace the incremented serial number.
-- This trigger function emulates the Informix DBMS behaviour.
--
-- The NEW variable contains the data for the row to be INSERTed or
-- UPDATEd. It, and its complement OLD used for UPDATE and DELETE,
-- are automatically created and populated by PostgreSQL whenever
-- a data-change function is called.
--
IF NEW.customer_num = 0 THEN
SELECT nextval('customer_customer_num_seq') INTO NEW.customer_customer_num;
ELSE
IF NEW.customer_customer_num > 0 THEN
PERFORM setval('customer_customer_num_seq', NEW.customer_customer_num);
END IF;
END IF;
RETURN NEW;
END;
$$ LANGUAGE plpgsql;

ALTER FUNCTION public.customer_num_informix() OWNER TO hll_4gl_testing;

CREATE TRIGGER customer_num_serial
BEFORE INSERT ON customer
FOR EACH ROW EXECUTE customer_num_informix();

The problem is that I am getting a syntax error on the CREATE TRIGGER statement:

ERROR: syntax error at or near "customer_num_informix"
LINE 3: FOR EACH ROW EXECUTE customer_num_informix();

I do not see what the error is. What is wrong with the syntax I used?

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