Diagnosis
The total of the quotients (total of degree day coefficients /conversion factor) overall the months of the interval is zero.
In this case, no weighted average of conversion factors can becalculated in the gas billing, since this number is used in thedenominator in the relationship (total weight/total of quotients).

Procedure
Please check the degree day coefficient tables, as well as the gastables for calorific value and volume correction factor.
Usually, both the degree day coefficients and the conversion factorsmust be positive, real numbers.