Issue #264 resolved
When I have a cell formatted as a date containing the formula
the optimized reader fails with
Traceback (most recent call last): File "<stdin>", line 1, in <module> File "C:\Python27\lib\site-packages\openpyxl\reader\iter_worksheet.py", line 219, in get_squared_range cell = cell._replace(internal_value=self._shared_date.from_julian(float(cell.internal_value))) ValueError: could not convert string to float: =""
The regular reader works without any problems and gives None for that cell as I would expect.