Null parm to a function

I want to be able to pass NULL as a parameter to a function I've defined,
but I'm not allowed to do it. I believe this is a bug.

The example follows:

CREATE TABLE test_func(                     func_sun      int4, surname       varchar(30), value         int2
NOT NULL,  CONSTRAINT p01_test_func PRIMARY KEY(func_sun)
 
);

This insert works as one would expect.

INSERT INTO test_func(func_sun, surname, value)
VALUES(1234,null,99);

Now create the function that will insert rows into my table test_func.

CREATE FUNCTION insert_row(int4,varchar,int2)
RETURNS int4 AS 'INSERT INTO test_func(func_sun, surname, value)  VALUES($1,$2,$3);  SELECT 1 AS ignore;'LANGUAGE
'sql';

Now if I try to do the same thing but via my function,SELECT insert_row(1235,null,100);
I get the following errorERROR:  typeidTypeRelid: Invalid type - oid = 0

Why wont the function allow me to pass a 'null' parm?

Cheers...Gordon
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