functor

I thought it would be easy and convenient to define a small functor and perform a customized sort on some objects I have. However, I can't seem to get the functor to work. Here is some code that gives me a "no matching call" error:

class Cmp
{
    bool operator()(int x, int y) const
    {
        return x < y;
    }
};

vector<int> vec;
std::sort(vec.begin(), vec.end(), Cmp() );

As far as I can tell I followed the requirements exactly, but apparently I didn't. Can anyone tell me what is wrong with the above code? Thanks for your time.