ZoHo deluge time difference subform calculation error

ZoHo deluge time difference subform calculation error

Hello,

I am using the below script to calculate the time difference for every entry in a subform

row.Time_Sheet.Decimal=((row.Time_Sheet.End_Date_Time_of_task - row.Time_Sheet.Start_Date_Time_of_task)) / (1000 * 3600);

however I am getting the "Variable Time_Sheet not defined"Time_Sheet is the name of the subform, how do I correct the error