New Post: comment shape

Hello,
I cannot control the shapes of comments created with the following code:

ICell cell = sheet.GetRow( row ).GetCell( column );
IDrawing drawing = sheet.CreateDrawingPatriarch( );
XSSFClientAnchor anchor = new XSSFClientAnchor( 0, 0, 0, 0,
    cell.ColumnIndex, cell.RowIndex, cell.ColumnIndex + 10, cell.RowIndex + 3 );
IComment comment = drawing.CreateCellComment( anchor );
comment.String = new XSSFRichTextString( "my comment" );

One comment has a rectangle shape, another rounded rectangle, then triangle, oval etc.
How can I force a rectangle shape for each comment?