Initialize AdvWebGrid cell with edCombo Editor

I am trying to programmatically set the value of a cell in a column with Editor set to edCombo.

When I initialize the grid the cell displays the correct value. When I turn on edit mode the cell becomes blank despite the value being present in both ComboItems and ComboValues lists. It seems the edcombo also does not give me the option to type free text if the desired value is not in the list of items.

How can I correctly initialize a combobox.