strange behavior with C function and DEFAULT function parameters

Hi,

I ran into some pretty strange behavior of C-language function and
default parameter values, both on 9.2 and 9.4devel. Consider for example
this trivial C function:

Datum
show_bug(PG_FUNCTION_ARGS) {
elog(WARNING, "called ;-)");
PG_RETURN_VOID();
}

which is accessed using this definition:

CREATE FUNCTION show_bug(a TEXT DEFAULT NULL)
RETURNS void
AS 'bug.so'
LANGUAGE C STRICT;

and let's try various calls:

db=# SELECT show_bug('a');
WARNING: called ;-)
show_bug
----------

(1 row)

Seems ok. Now let's use the default value:

db=# SELECT show_bug();
show_bug
----------

(1 row)

db=# SELECT show_bug(NULL);
show_bug
----------

(1 row)

Well, seems quite strange to me - it seems as if the function is called,
but apparently it's not. I can't find anything relevant in the docs.

For comparison, a matching PL/pgSQL function:

CREATE FUNCTION show_bug2(a TEXT DEFAULT NULL) RETURNS void AS $$
BEGIN
RAISE WARNING 'called ;-)';
END;
$$ LANGUAGE plpgsql;

which behaves exactly as expected in all three cases:

db=# SELECT show_bug('a');
WARNING: called ;-)
show_bug
----------

(1 row)

db=# SELECT show_bug();
WARNING: called ;-)
show_bug
----------

(1 row)
db=# SELECT show_bug(NULL);
WARNING: called ;-)
show_bug
----------

(1 row)

So, what I'm doing wrong? Seems like a bug to me ...

regards
Tomas