TypeError: 'NoneType' object is not a mapping code example

Example: TypeError: 'NoneType' object is not iterable

def write_file(data, filename): # creates file and writes list to it
  with open(filename, 'wb') as outfile:
    writer = csv.writer(outfile)
    for row in data: # ABOVE ERROR IS THROWN HERE
      writer.writerow(row)