Unable to borrow bookcopy

Internal Server Error: /app/exlibris/books/borrowing/person
Traceback (most recent call last):
  File "/home/dolphi/Projects/AlekSIS/venv/lib/python3.7/site-packages/django/core/handlers/exception.py", line 34, in inner
    response = get_response(request)
  File "/home/dolphi/Projects/AlekSIS/venv/lib/python3.7/site-packages/django/core/handlers/base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "/home/dolphi/Projects/AlekSIS/venv/lib/python3.7/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/lib/python3.7/contextlib.py", line 74, in inner
    return func(*args, **kwds)
  File "/home/dolphi/Projects/AlekSIS/venv/lib/python3.7/site-packages/django/contrib/auth/decorators.py", line 21, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/home/dolphi/Projects/AlekSIS/AlekSIS/apps/official/AlekSIS-App-Exlibris/aleksis/apps/exlibris/views.py", line 294, in person_borrow
    book_copy.borrow_count = book_copy.borrow_count + 1
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'