.check_fixity() does not work if fixity does not exist

[ghukill]

In [21]: obj.check_fixity()                                                                                                                                                                                                                                                                
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-21-c6745659dca2> in <module>
----> 1 obj.check_fixity()

~/pyocfl/pyocfl/pyocfl.py in check_fixity(self, fixity_algo, use_manifest_digest)
   1088 
   1089                 # get fixity digests
-> 1090                 fixity_old = self.object_inventory.fixity.get(fixity_algo,None)
   1091 
   1092                 if fixity_old != None:

AttributeError: 'NoneType' object has no attribute 'get'