When a token's head does not have a part-of-speech tag, it is reported as if the token itself not have a tag. This error could be improved in two ways:
Print the full sentence, so that it is easier to find the problematic token.
Adjust the message to state that the token's head does not have a POS.
When a token's head does not have a part-of-speech tag, it is reported as if the token itself not have a tag. This error could be improved in two ways: