Report Template, custom grouping doesn't work with custom parameters

Attempting to add a custom grouping to a Report Template via code did not work if the Report Template had custom Parameters.

For example, the following code would work if the Report Template used default Parameters but not with custom Parameters:

Select Case eventId
  Case "AfterParametersFinalise"
      If Not Parameters.GetGroupingItems("Groupings").Exists("CustomGroup") Then
        Parameters.GetGroupingItems("Groupings").Add("CustomGroup", False, "Group")
      End If
End Select