SSRS Table Report Filter Problem

I am using VS 2005/SQL 2005 and must use a stored procedure as provided. The stored procedure returns parent rows and child rows (same as parent with additional columns populated, like ChildID). I cannot change the stored procedure.

I have been unable to filter the data (either at the Dataset level or the Table Properties level) to only see the "parent" rows. On my report, I added one column to print the value for my test and a second to use it:


=Trim(CStr(Len(Fields!ChiIdID.Value)))

=IIf(Trim(CStr(Len(Fields!ChildId.Value)) = "0"),"PARENT","CHILD")

Both columns produce the values I expect. However, if I try a filter using the first expression, it fails! I.E. if I test = 0, it filters out ALL rows and if I test != 0, all rows are displayed?????

I am new to SSRS, so I would not be surprised if this is cockpit error. It seemed like a very simple requirement that I have used in othe report tools for years.

Any help would be appreciated.