Problem retrieving data from a datagrid view drop down cell

I have a drop down cell within a data gridview where the user can select different names of supervisors. Once the default selection in the drop down is changed and the cell end edit is fired, the data is not being updated as far as I can tell. I debugged the code below and when it gets to the last line the valuer for cell_suprev is showing Nothing (on the screen it is showing correctly). What am I missing? I am using Visual Basic code in VS 2012.

        Dim rowindex As Integer = DataGridView_Supervisor.CurrentCell.RowIndex
        Dim row As DataGridViewRow = DataGridView_Supervisor.Rows(rowindex)
        Dim cell_superv As DataGridViewCell = row.Cells(4)
        Dim supervname As String = cell_superv.Value