Django- Reversing custom Admin URLs

I'm using an App that configures some Admin URLs as:

#admin.py....
    def get_urls(self):
            urls = super(FormAdmin, self).get_urls()
            extra_urls = [
                url("^(?P<form_id>\d+)/entries/$",
                    self.admin_site.admin_view(self.entries_view),
                    name="form_entries"),
            #......
            ]
            return extra_urls + urls

I'm having trouble using template tags to get a URL corresponding to that, in one of my templates. I'm trying stuff like:

<a href="{% url 'admin:forms_form_entries' form_id=4 %}">4-Entries</a>

(Where forms is the App's label). I keep running into the No Reverse Match type of errors:

NoReverseMatch at /polls/ Reverse for 'forms_form_entries' with arguments '(4,)' and keyword arguments '{}' not found. 0 pattern(s) tried: []

What am I missing that makes the tag work correctly?