Cumulative Sum Question

I want to calculate cumulative sum for column ‘Net Weight’ by Year . Below picture is result i want to have.

Column definition:

1. ‘SO Created Year’ = ROUND_YEAR("VBAK"."ERDAT")
2. ‘Created Month’ = ROUND_MONTH("VBAK"."ERDAT")
3. ‘Net Weight’ =  KPI(“so_netweight”)

4. Cumulative Sum=

         if I use formula : RUNNING_SUM(kpi("so_netweight"),order by("vbak"."so_creatd_year"), partition by("vbak"."created_month_so_"))  , i got below error.
 

can anyone help ?