Question About Using OldValuesParameterFormatString

I read on the MSDN about how to use the OldValuesParameterFormatString propery to access original parameter values in the sql statements of the SqlDataSource.  I used this in the following code:
  <asp:SqlDataSource ID="PlannedFeaturesData" runat="server"
ProviderName='System.Data.SqlClient'
ConnectionString="<%$ ConnectionStrings : SiteData %>"
OldValuesParameterFormatString="old_{0}"
SelectCommand="SELECT * FROM ProjectPlannedFeatures WHERE ProjectID=@ProjectID ORDER BY SortID"
DeleteCommand="DELETE ProjectPlannedFeatures WHERE SortID=@SortID AND ProjectID=@ProjectID"
UpdateCommand="UPDATE ProjectPlannedFeatures SET
SortID=@SortID,
Text=@Text,
Status=@Status
WHERE SortID=@old_SortID AND ProjectID=@ProjectID"
InsertCommand="INSERT INTO ProjectPlannedFeatures
(SortID, ProjectID, Text, Status)
VALUES
(@SortID,@ProjectID, @Text, @Status)">
</asp:SqlDataSource>From what I can tell I've implemented this properly, but I get the following error after click on the Update button on a GridView bound to this data source:
Description: An
unhandled exception occurred during the execution of the current web
request. Please review the stack trace for more information about the
error and where it originated in the code.



Exception Details: System.Data.SqlClient.SqlException: Must declare the scalar variable "@old_SortID".
 Any ideas?