Report Builder - Finding Specific Data Inside Of A LookupSet

Report Builder 3.0 . I have two datasets and I am using LookupSet to return data (a list of dates) but what I am after is the LAST date in the set. I have tried to use Last but I keep getting an error. 

=Last(LookupSet(Fields!Model_Name.Value,Fields!Model_Name.Value,Fields!Read_Date.Value,"dsPageCount"))

I even tried doing a Last on the above statement with a Join thrown in, but that just gets me errors as well. I tried to Code in a function to go through the dataset and return the last piece of data, but again...error. 

Function LastLookup(ByVal items As Object()) As Date
If items Is Nothing Then
Return Nothing
End If
Dim suma As Date = New Date()
For Each item As Object In items
suma = CDate(item)
Next