id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
17179	yesno arg is optional	CarlFK	nobody	"
""Given a string mapping values for true, false and (optionally) None, returns one of those strings according to the value:""   
https://docs.djangoproject.com/en/dev/ref/templates/builtins/#yesno 

Looking at the code, I see the string is optional.

    if arg is None:
        arg = ugettext('yes,no,maybe')

http://code.djangoproject.com/svn/django/trunk/django/template/defaultfilters.py
"	Cleanup/optimization	closed	Documentation	1.3	Normal	fixed			Accepted	0	0	0	0	1	0