Why does "No. of Days in US on Business" have to be at least 1?

I've lived abroad for years and am applying for the foreign income exclusion like usual. This time though, when I am describing the trip I took back to America for Christmas it says the minimum value of "No. of Days in US on Business" must be at least 1. None of those days were on business because....it was Christmas and I was with my family. What does this mean?