ClientDataSet and TAggregate

I am using a clientdataset as a memory-only temp table (no provider or
dataset, structure created with TFieldDefs).  It is working great, but now I
want to add an aggregate to sum all of one column.  I double clicked on the
Aggregates property and defined an aggregate for the Payment column in the
dataset.  AutoAggregate is set to true, the Aggregate is enabled, visible,
and has the expression Sum(Payment).  The grouplevel is 0 and the index is
blank.  Whenever I try to access the aggregate, its InUse is always false
and the Value is null.  I can't figure out why.  I've looked at all the help
files and as far as I can tell I am doing it right (I want to sum the entire
Payment column, no grouping, or anything like that).  Can someone point out
what I am doing wrong?  Thanks!

Ryan McGinty