Stdev with missing data sets in Qlik sense

[marc_qlik]

Hello,

I have a data set with 100 materials and with the corresponding number of pieces on a daily base. Now I want to calculate the stdev on a monthly base for each material. Problem is, that not for each month a data set exists  (s. example row 1).#

Can the formular be fixed?

Stdev(aggr(sum(pieces), [Material], [Month]

1. row: Stdev of 26,904 in Qlik with missing data sets for Apr, Jun, Aug and Oct.

2. row: Stdev as it should be 24, 772 with "0" instead od NULL

Thank you