Regression failure - but why?

I'm trying to run a regression analysis on the data, but everytime I include December, it somehow fails with #NUM on the regression.

If I exclude december tho, it success, but I need the last month in order to make the function.

I have run a value() on all the numbers to make sure it's numeric.

I have success with these combinations:

Sales (Y) + C20 (X) + Temp (X) +Months -December (X) => Works fine!
Sales (Y) + All months (X) => Works fine!
Sales (Y) + All variables (X) => Doesn't work


Can anyone tell me why?