KeyError as a direct cause of exception when i try to insert values where there is None

Question

Goal: Whenever there is None, the values of three should be inserted

df3.loc[df3['wp'].isnull(), 'wp'] = df3['three']

works for most of the data but somewhere (i cannot determine where) it throws

KeyError: 'wp'

Question: How can i try:except or work around this error in order to keep the code running until all data has been processed?