Formula app doesn't work in the first entry

[seri77]

Hi, I am experiencing the following problem:

I want to calculate the total hours worked per week in my timesheet.

In the column of total hours/week I put the following formula in the app:

SUM(SELECT(Timesheet[Total Hours],[Settimana num]=[Week Num])).

This formula is running in the spreadsheet only from the second entry. In the first entry, the result is 00:00:00 even if total hours are 08:00:00.

Is someone able to help? Maybe is the formula wrong? 

I also tried to put the inizial value, but nothing, it doesn't work.

Thanks a lot

Serena