Error when running fasttagcount without cb_histogram

Hi,
I am currently testing umis for our 10XGenomics data. fasttagcount crashes when no cb_histogram is provided:
  File "build/bdist.linux-x86_64/egg/umis/umis.py", line 829, in fasttagcount
AttributeError: 'NoneType' object has no attribute 'index'

There seems to be no check at that point whether cb_histogram was actually TRUE or whether cb_hist was not NONE.

Best, Thomas