Error from xlsm file when adding strings to a xltm template
*Created originally on Bitbucket by [egene (Elizabeth Gene)](https://bitbucket.org/%7Befb403b1-a0b4-4738-a30c-a0a52f436661%7D/)*
This sample code:
```
#!python
from openpyxl import load_workbook
wb = load_workbook('template.xltm', keep_vba=True)
ws = wb.active
cell = ws.cell(column=1, row=1, value='42')
wb.save('sample_book.xlsm')
```
produces a corrupted spreadsheet when run on Ubuntu with these versions:
* Python 2.7.6
* lxml Version: 3.5.0
* openpyxl Version: 2.3.3
If the value is changed to an int, the spreadsheet is fine.
*Attachments:* [sample_book.xlsm](/uploads/1ff249599055d49455d3f933a2f5618f/sample_book.xlsm)[template.xltm](/uploads/e4a8166c7af77f4d63397cf7797e99da/template.xltm)[Bildschirmfoto_2016-01-29_um_17.05.42](/uploads/55d31a2ddf11808faabf1fbdd842149f/Bildschirmfoto_2016-01-29_um_17.05.42.png)
issue