Python 2.5 Modules/readline.c line 681 is:

	return completion_matches(text, *on_completion);

on_completion is a function declared as:

static char *on_completion(char *text, int state);

completion_matches is a macro that expands to: 

rl_completion_matches((x), ((rl_compentry_func_t *)(y)))

SO, the second parameter to completion_matches should be a function pointer (on_completion), not the dereferenced function pointer that is currently passed to it (*on_completion).