Trouble finding certain tag values

I'm trying to find the value of several tags using pydicom. For some reason only certain tags work while others do not. Below is a traceback explaining my problem. Can anyone see a way around the int() base 16 problem?

>>> ds['0x18','0x21'].value
'SP'
>>> ds['0x18','13x14'].value
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/space/jazz/1/users/gwarner/anaconda/lib/python2.7/site-packages/pydicom-0.9.9-py2.7.egg/dicom/dataset.py", line 276, in __getitem__
tag = Tag(key)
  File "/space/jazz/1/users/gwarner/anaconda/lib/python2.7/site-packages/pydicom-0.9.9-py2.7.egg/dicom/tag.py", line 27, in Tag
arg = (int(arg[0], 16), int(arg[1], 16))
ValueError: invalid literal for int() with base 16: '13x14'