plpgsql and intarray extension; int[] - int[] operator does not exist ?

I have installed the intarray extension installed in the public schema and am attempting to use this in a plpgsql trigger function from another schema.
When the triggers executes this I get an exception to the effect

{
"hint": "No operator matches the given name and argument type(s). You might need to add explicit type casts.",
"details": null,
"code": "42883",
"message": "operator does not exist: integer[] - integer[]"
}

However, If I write a similar test function and attempt similar array arithmetic successfully from a different schema in a non-trigger function It recognizes the intarray methods.

CREATE OR REPLACE FUNCTION admin.djd_test()
RETURNS integer[] AS
$BODY$
DECLARE
_old_tag_ids INTEGER[];
_new_tag_ids INTEGER[];

BEGIN
_old_tag_ids := ARRAY[1,2,3];
_new_tag_ids := ARRAY[3,4,5];

RETURN _old_tag_ids - _new_tag_ids;

END;
$BODY$
LANGUAGE plpgsql VOLATILE

Of course in the trigger function the declared int[] arrays the content is dynamically initialized.
Any suggestions as to why the int[] operations are not understood in the trigger context.?

.
Thanks

Dave Day