error when read float pixel from a Mat

Hi everyone, I just want to read a value from each pixel of the oIMG mat, but it is failed. Can you help to explain where is my errors in below code: Mat oIMG[3]; for (int i = 0; i < 3; ++i) { oIMG[i] = Mat(vSize, hSize, CV_32FC(1), Scalar::all(255)); } int hh = oIMG[0].at(0, 0);