Why does cv::imwrite is not working when I include the .png in the back?

I ran into this error message when I ran the following code:

           char fileName[60];

            //write new filname
            sprintf(fileName,"/home/pi/Desktop/ML_TakeSample/DistanceMeasurement/image%i.png",i);
            cout<<"saving "<< fileName <<endl;

            //prep parameters for PNG file
            vector<int> compression_params;
            compression_params.push_back(IMWRITE_PNG_COMPRESSION);
            compression_params.push_back(9);

            //save to folder
            imwrite(fileName, ToBeSaved[i], compression_params);

the error message:

error: (-2:Unspecified error) could not find a writer for the specified extension in function 'imwrite_'

When I look at others code that had the same error message, they didn't include .png or .jpg