The problem of the mask?

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I don't know how to use the mask. Some problem has occur when i use the following codes,but i don't know why. please help me to understand.thank you!

#include<iostream>
#include<opencv2/core/core.hpp>
#include<opencv2/highgui/highgui.hpp>

using namespace std;
using namespace cv;
int main()
{
    Mat_<float> m(3, 3);
    m << 1.0f, 2.0f, 3.9f, 4.0f, 5.0f, 6.0f, 7.7f, 8.0f, 9.0f;

    Mat_<int> mask1(3, 3);
    mask1 << 1, 2, 3, 0,1,2,3,0,0;

    Mat m2;
    m.copyTo(m2,mask1);

    cout<< m2 << endl;

       system("pause");
}