I noticed that while the selName argument is optional with a default value of 'foundSeqXYZ', not specifing it gives rise to this error:
Error: selName was not a string.
This is because when the function seqFinder is called, is it set to None and then, the checkParams function makes sure that it is a string. Changing line 48 to:
if type(selName)!=(types.StringType) and type(selName)!=(types.NoneType):
solves the problem.