calculate_dimension() shows wrong end, but I can't set attribute!?

[mgaldzic]

ws.calculate_dimension(). this returns a range that I know is incorrect.

I tried setting it as suggested in the docs[1].
ws.max_row = ws.max_column = None

Returns:
AttributeError: can't set attribute

[1] http://openpyxl.readthedocs.io/en/default/optimized.html?highlight=dimension#worksheet-dimensions