Need help with Drag-and-drop works for muilti rows but not single

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Hi, I'm using vs2005, .net 2 for a window application. My datagridvew was
working before when I drag just one item to another datagridvew. I modified
the code to do multiple rows draging. Now when I drag multiple rows it works
but when I drag one row I get this circle and a diagnoal line in it, I think
that's a "No" sign, when I'm draging to the target grid. When I release the
mouse button then nothing happen.
I track this to the Datagridview_MouseMove event where the DragDropEffects
variable is alway returned as null when I drag just one item:

DataRowView view = (DataRowView) dgv.Rows[info.RowIndex.DataBoundItem];
dragDroip = dgv.DoDragDrop(view, DragDropEffects.All);

Does anyone know what could be cuasing this? Thanks.
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Thanks.