This blog post shows an example of how to create a immutable_concat function in Pg:

CREATE OR REPLACE FUNCTION immutable_concat(VARIADIC "any")
  RETURNS text AS 'text_concat'
  LANGUAGE internal IMMUTABLE

I'd like to do the same with concat_ws and the corresponding text_concat_ws does exist, however, the following just crashes the process:

CREATE OR REPLACE FUNCTION immutable_concat_ws(VARIADIC "any")
  RETURNS text AS 'text_concat_ws'
  LANGUAGE internal IMMUTABLE

Update: The siguature of immutable_concat_ws should be (glue, *parts), one glue (text or varchar) and one or more parts (text, varchar or null).

What am I missing here?