Help with SSRS 2005 Expression (If statement)

Ok, this is weird.  When you are in the expressions editor, if your report has more than one dataset defined like mine (because I'm doing some other calcs), then you see this in the left pain:
Fields(DataSet1)

Datasets

What I was doing was using the field values in my DataSets area...in it it had an additional "DataSet1" option for some reason, even though it was already showing Fields(DataSet1) outside the Datasets option on the left.  

So when using the DataSet1 fields inside Datasets, I was in turn adding fields to my IIf statement that were actually in the report already...they were preceeded by SUM (because they're in a group) so this is not going to work obviously.

So I realized, I had to use the Fields(DataSet1) fields which were my original fields and then changed it to this:

=IIf(Fields!New_Old_CC.Value = 1, SUM(Fields!PDC.Value),0)

Ok, the report rendered...some were summed but in some of the fields for that column I got an error:

[rsRuntimeErrorInExpression] The Value expression for the textbox ‘textbox86’ contains an error: Input string was not in a correct format.

So I realized, that there are cases where New_Old_CC is null so I need to somehow check this...maybe do an IIf inside my IIf to check IsNothing I guess?

=IIf(IIf(Not(Isnothing(Fields!New_Old_CC.Value) AND Fields!New_Old_CC.Value = 1), Fields!PDC.Value,0)

not sure how to form this