We have found that in some cases the astrodrizzle task throws the error stated above. Here is a quick solution found by the Drizzlepac team that users can implement while the code is fixed in the distribution. This is a quick fix that has not been tested in any way. USE AT YOUR OWN RISK.
In the file /path/to/python/packages/lib/python2.7/site-packages/stwcs/wcsutil/altwcs.py changes lines starting at 352 from:
for k in hwcs:
for k in hwcs.keys():