SSRS expression returns #error value in some cells

Hello,

What do you think below expression sometimes return #error value in ssrs.

=SUM(Fields!X.Value / 100 * DateDiff(DateInterval.Day,Fields!s_date.Value,Fields!e_date.Value))  / 
IIF( SUM(DateDiff(DateInterval.Day,Fields!s_date.Value,Fields!e_date.Value)) = 0
	 ,nothing
	 ,SUM(DateDiff(DateInterval.Day,Fields!s_date.Value,Fields!e_date.Value)) 
    )