how to set a Mat object value with anther Mat

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the definition of setTo function is as follows:

Mat& setTo(InputArray value, InputArray mask=noArray());

According to the definition, there is a running exception when I use the function like this:

a_mat.setTo(b_mat);//a_mat and b_mat has the same type and size

why this happened? If I want to set a Mat value to another using setTo function, is it practicable？