OpenCV ConvertTo CV_32SC1 from CV_8UC1

Hello I am using opencv in version 3.4 and want to read an image (*.pgm) and then convert it to CV_32SC1. Therefore I use the following code (part):

img = imread(f, CV_LOAD_IMAGE_GRAYSCALE);
img.convertTo(imgConv, CV_32SC1);

The problem is the following, all pixels are converted to zero, and I don't understand why. I'm checking by (and imshow("Image", imgConv);)

cout << static_cast<int>(img.at<uchar>(200,100));
cout << static_cast<int32_t>(imgConv.at<int32_t>(200,100)) << endl;

In my example this results in

74 74

I tested several points of the image, all pixels are simply the same, but shouldn't them being converted automatically to the 32 bit range, or do I have to manage that manually?