passing arg 2 of `g_timeout_add' from incompatible pointer type

[Yiannis <odysseus lost gmail com>]

OK, I always get the following warning and I never understood why.
 passing arg 2 of `g_timeout_add' from incompatible pointer type

A typical example of my code would be:

gboolean
foo(void)
{
return TRUE;
}

g_timeout_add(500, foo, NULL);

Any clues?
Thanks!

-- 
-- Yiannis