VMLDrawing part

Hi,

The code part  VmlDrawingPart vmlDrawingPart = worksheetPart.AddNewPart<VmlDrawingPart>(); from the below given link is not working for me.I'm getting an exception saying that "only one instance of the type is allowed for this parent". Can anyone help??

http://social.msdn.microsoft.com/forums/office/en-US/40a95862-9adc-492d-a046-97a5e6e20260/how-to-insert-comments-in-excel-using-openxml

Thanks