Getting #Error with aggregate function

Why doesn't this return a value?
=Sum(IIf(Fields!GroupCode.Value = 10, Fields!Rating.Value, 0))
I am getting #Error as the value. This however gives me a value:
=Sum(IIf(Fields!GroupCode.Value = 10, 1, 0))
However, I need the above to work...I need it to sum the rating if it's
part of a particular group.Here are some additional findings...
The following code works:
=Sum(IIf(Fields!GroupCode.Value = 10, CInt(Fields!Rating.Value), 0))
The following code fails:
=Sum(IIf(Fields!GroupCode.Value = 10, 1.20, 0))
=Sum(IIf(Fields!GroupCode.Value = 10, CDbl(Fields!Rating.Value), 0))
Why is it that it can only sum up intergers?|||I figured it out!!
The following code works:
=Sum(IIf(Fields!GroupCode.Value = 10, 1.20, 0.0))
Both the true and false values need to be of the same type. By return
0 as my false condition value, and 1.2 as my true condition, it caused
it to fail because it's returning different data types base on the
different condition.
This is bad, MS needs to fix this.