Multimap no match for operator==

I have this multimap inside a Bar class, with an unsigned long and a reference to the Foo class.

class Bar
{
//etc.
        struct map_sort
        {
            bool operator()(const unsigned long& e1, const unsigned long& e2) const
            {
                return (e2 < e1);
            }
        };

    std::multimap<const unsigned long,const Foo&, map_sort> m_map;

    find_and_erase(const unsigned long var1, const Foo& var2);
}

And now I want to retrieve all the values from the multimap and erase some.

void Bar::find_and_erase(const unsigned long var1, const Foo& var2)
{
    auto it = m_map.begin();

    for (it=m_map.begin(); it!=m_map.end(); ++it)
    {
        const unsigned long first = (*it).first;
        const Foo& second = (*it).second;
        if((first == var1) && (second == var2)) //<----ERROR No Match for operator==
            m_map.erase(it);
    }
}

Question

How can I compare the (second == var2) ?

(What I want is to find all entries from a multimap, and delete the ones on the find_and_erase() function, that matches the function argument. Don't know if there is an easier solution for this)