I keep getting a type error in my Python caller

I keep getting an error in my Python caller transformer trying to convert dates in the format '%Y%m%d' to Python 'datetime.datetime' format. The transformer works for the first 236 features and generates the following error:

Python Exception <TypeError>: strptime() argument 1 must be str, not None

Traceback (most recent call last):

File "<string>", line 38, in input

TypeError: strptime() argument 1 must be str, not None

Error encountered while calling method `input'

I have examined the input data and nothing seems out of place or wrong with the data. I am not sure what's causing this.