Average error iin calculated field of Excel.

[devpp]

Hi guys,
I have made a pivot for a sample data in "Audit Scores" sheet.
The labels are January week 1,week 2,week 3,week 4 and week 5.
The pivot table calculates the monthly average in "A-Month" sheet for different months.
The average value is correct when there are 5 or 4 values depending on the months.
But when there is blank value, the blank cell is taken as zero and hence the result is wrong.
Can anyone tell me a solution for this.
I have attached the sheet for details.
The cell with error value is
C34 in A-Month sheet.
The value is taken from row 197 from Sheet Audit Scores.
Thanks and
Regards,
Devpp.