A simple problem of openCV(mul function)

[stereoMatching]

Mat_<double> const affine = (Mat_<double>(2, 2) << 0.5, 1, 
                                                     0,   2);

  Mat_<double> coordinate = (Mat_<double>(2, 1) << 20, 
                                                   3);
  Mat_<double> C(2, 1);

  C = affine.mul(coordinate); // this line would crash the program

  C = affine * coordinate; // this line is ok 

I don't why and how, did I did something wrong?
Thanks a lot