cannot convert ‘cv::Mat’ to ‘float’

edit

I want to multiply a 1x6 with a 6x1 Mat types matrices, and then store it in a matrix entry as follows;

Vy.at<float>(i,j) = fy.rowRange(i,i+1) * a.colRange(n, n + 1);

But if I compile this I get error: cannot convert ‘cv::Mat’ to ‘float’ in assignment

Does anybody know how to solve this?