Instead of
=IIF(Sum(Fields!Denominator.Value) > 0, Sum(Fields!Numerator.Value)/(Sum(Fields!Denominator.Value), 99.9)
I used
=SWITCH( Sum(Fields!Denominator.Value) = 0, 99.9, Sum(Fields!Denominator.Value) <> 0, Sum(Fields!Numerator.Value)/(Sum(Fields!Denominator.Value) )