Trouble with operator [] in Debug Window VisualStudio

I have a c++ class of matrices with only one field:

template <class T>
class matrix {
private:
    std::vector < std::vector <T>> elements;
public:
    ...
}

Also I have overdriven operator []:

    std::vector <T>& operator[] (int index) {
        return elements[index];
    }
    const std::vector <T>& operator[] (int index) const {
        return elements[index];
    }

And I can use it in code

    matrix <double> A(5,5);
    A[1][3] = 5;

But I can't add such expressions to debug window. I add A[2] or A[2][3] and get no operator "[]" corresponding to these operands. How can I fix this?