AttributeError: 'NoneType' object has no attribute 'upper'

[shayanb]

When running python3 whatsBook.py ExportedChat > content.tex fails with the following error:

Traceback (most recent call last):
  File "whatsBook.py", line 385, in <module>
    main()
  File "whatsBook.py", line 78, in main
    for line in parsedChat:
  File "whatsBook.py", line 317, in parseChat
    img = drawMonth(img, prevMonth, mask=False)
  File "whatsBook.py", line 325, in drawMonth
    width, height = draw.textsize(month.upper(), font)
AttributeError: 'NoneType' object has no attribute 'upper'