How can I summarize the result of Lookup values in SSRS 2008?

I've a SSRS Report where one column is a result of Lookup from another dataset. I need to show the total of this column.

When I create a total for a group the column is doesn't include in my total, I think this is because is a result of the lookup data from another dataset?

For each row I'm using the following expression: =Lookup(Fields!InstallerID.Value, Fields!InstallerID.Value, Fields!RoutedPoints.Value, "Lookupdataset"), the return value is correct

And for a total row I'm using: =Sum(Lookup(Fields!InstallerID.Value, Fields!InstallerID.Value, Fields!RoutedPoints.Value, "Lookupdataset"), "Maindataset")

But the value is wrong.

How can I get the sum of this column?

I tried to use my own code but it's work for the first total only, becuase how it's not part of the group this is not clear !!!

Thanks in advance

Eliana