When it asks you how much of your income was earned from the Home Office, saying less than 100% causes that problem (it often causes line 8 of Form 8829 to become negative).
Personally, I think it is an error in the law, but unfortunately, that is how the law is written. Although some people think a person should say 100% in all circumstances (unless they have multiple offices), but my first inclination is that isn't right.
