ValueError when attempting to update a listitem field which current value is null

[RogerPoller]

I am trying to update an existing field of a listitem which current value is null (that means it's not listed on item.fields) , but the following exception happens:

ValueError: "[FIELD NAME]" is not a valid internal field name

I have tried updating other field (which value wasn't null) of the same item and it works fine. I have also checked that I am using the correct internal field name so that's not the cause of the problem as the message error suggests.

This is my first time reporting an Issue, so excuse me if I am not providing enough information to analyze this problem any further, if that's the case, please let me know.

[alejcas]

A bug maybe, I don't have the time right now to investigate this. I'm sorry.