Opening .xlsx on OS X crashes
*Created originally on Bitbucket by [gmagill (Jerry Magill)](https://bitbucket.org/%7Bd091e2c6-86d9-4861-b757-86da31db8da9%7D/)*
Trying to open a Excel workbook but I get an instant crash:
```
#!python
workbook = load_workbook(datafile)
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/openpyxl-2.5.0b1-py2.7.egg/openpyxl/reader/excel.py", line 274, in load_workbook
for c in find_charts(archive, rel.target):
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/openpyxl-2.5.0b1-py2.7.egg/openpyxl/chart/reader.py", line 43, in find_charts
drawing = SpreadsheetDrawing.from_tree(tree)
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/openpyxl-2.5.0b1-py2.7.egg/openpyxl/descriptors/serialisable.py", line 83, in from_tree
obj = desc.expected_type.from_tree(el)
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/openpyxl-2.5.0b1-py2.7.egg/openpyxl/descriptors/serialisable.py", line 83, in from_tree
obj = desc.expected_type.from_tree(el)
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/openpyxl-2.5.0b1-py2.7.egg/openpyxl/descriptors/serialisable.py", line 83, in from_tree
obj = desc.expected_type.from_tree(el)
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/openpyxl-2.5.0b1-py2.7.egg/openpyxl/descriptors/serialisable.py", line 83, in from_tree
obj = desc.expected_type.from_tree(el)
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/openpyxl-2.5.0b1-py2.7.egg/openpyxl/descriptors/serialisable.py", line 83, in from_tree
obj = desc.expected_type.from_tree(el)
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/openpyxl-2.5.0b1-py2.7.egg/openpyxl/descriptors/serialisable.py", line 99, in from_tree
return cls(**attrib)
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/openpyxl-2.5.0b1-py2.7.egg/openpyxl/drawing/shapes.py", line 147, in __init__
self.prst = prst
File "/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/openpyxl-2.5.0b1-py2.7.egg/openpyxl/descriptors/base.py", line 41, in __set__
if not isinstance(value, self.expected_type):
TypeError: isinstance() arg 2 must be a class, type, or tuple of classes and types
```
Code to reproduce bug:
```
#!python
from openpyxl import load_workbook
workbook = load_workbook("ucaas-datacollection.xlsx")
```
See workbook in attachment.
Many thanks.
*Attachments:* [ucaas-datacollection.xlsx](/uploads/8eb16e3f4d6a11ffb3d2cd9851f6add4/ucaas-datacollection.xlsx)
issue