#Error when performing a conditional Sum()

I am trying to perform a Sum() of time worked during the week and, seperately, during the weekend. The formula I am using for the weekday Sum() is:

=Sum(IIf((CInt(DatePart(DateInterval.WeekDay, Fields!Date.Value)) + FirstDayOfWeek.System) Mod 7 > 1, Fields!Time.Value, 0))

This results in #Error being displayed in the cell of the matrix. The issue seems to relate to the value of Fields!Date.Value because if I update the formula to:

=Sum(IIf((CInt(DatePart(DateInterval.WeekDay, Now())) + FirstDayOfWeek.System) Mod 7 > 1, Fields!Time.Value, 0))

Although this now assumes all activities were performed today, the result is a valid Sum() of the time spent. However I'm not sure what is wrong with the date data since extracting the "(CInt ... Mod 7" section of the expression into a new cell within the group it evaluates correctly for each column.

Can anyone advise what might be going wrong? The relevant subset of the dataset for this query is:

2016-04-25 00:00:00.000	7.5
2016-04-25 00:00:00.000	3.5
2016-04-25 00:00:00.000	0.5
2016-04-25 00:00:00.000	1
2016-04-25 00:00:00.000	7.5
2016-04-26 00:00:00.000	6.5
2016-04-26 00:00:00.000	6.5
2016-04-26 00:00:00.000	1
2016-04-26 00:00:00.000	7.5
2016-04-27 00:00:00.000	7.5
2016-04-27 00:00:00.000	4.5
2016-04-27 00:00:00.000	7.5
2016-04-27 00:00:00.000	1
2016-04-27 00:00:00.000	8
2016-04-28 00:00:00.000	6
2016-04-29 00:00:00.000	0
2016-04-30 00:00:00.000	0
2016-05-01 00:00:00.000	0

Update: Note that this issue occurs in the group's total column (where it's summing multiple days based on the condition I've included). Not sure if that was clear above.

Thanks.