doesntcompile

#include <cstdlib>
#include <iostream>
#include <vector>
using namespace std;

class A
{
public:

    A() : _bla(4)
    {

    }

    const int& operator()(int num1, int num2) const
    {
        return this->at(num1, num2);
    }

    int & operator()(int num1, int num2)
    {
        return this->at(num1, num2);
    }
private:

    int& at(int num1, int num2) const
    {
        return _bla[num1 + num2];
        // main.cpp:30:26: error: invalid initialization of reference of type ‘int&’ from expression of type ‘const value_type {aka const int}’
    }

    vector<int> _bla;
};

int main(int argc, char** argv)
{
    A a;
    cout << a(1, 1) << endl;
    return 0;
}