Can't Programmatically Select An Item In DataGridViewComboBoxColumn

I had a DataGridView, and already able to add DataGridViewComboBoxColumn to it. It read data from database StoredProcedure using custom ValueDescriptor class, this is the code for it:

Shared Sub fillComboBoxCellUsingSP(ByVal comboBox As DataGridViewComboBoxColumn, ByVal proc_name As String, ByVal param As Object(), ByVal firstitem As String)
Dim dt As New DataTable

[Code]....

Actually, the line that read comboBox.Value = o is okay. But still, the combo box cell didn't display the value. Just empty. And sometimes, DataError event raised.