Saving the scope shot to a local drive with 1kbyte sometime.

Hi, I am trying to save the scopeshot to a local computer with a python code. I generated two scripts to get a waveform For a bad code, it saves only 1kbyte size of the scope shot file with "unsupported formate". I attached two screen shots for a good case and a bad case. 

• Good case: Checking the code by using a break tool from pycharm. It saves the correct size of around 215kB with "imgData"

scope.write('FILESystem:READFile \"C:/Temp.png\"')
 imgData = scope.read_raw(1024*1024)
raw_data.append(imgData) 

• Bad case: Checking the same point. The file size is 6Byte as it can be shown in attached files. 

Do you have any comment to solve the bad case?