Diagnosis
A total value of zero was calculated as the weighting total of degreeday coefficients from the degree days table and the table of averagedegree day coefficients. This figure is used whenever the degree daycoefficient is not available for a specific day in the degree daystable.
It is likely that the degree days table was not maintained correctly.In this case, the weighted average of the conversion factors cannot becalculated consistently in gas billing.

Procedure
Please check both tables - particularly for the specified interval -and add any missing values.
Then start billing again.