Error in Matrix Column

I created a UDF field with valid value "Yes" and "No", and I added this column in Matrix of a standard form.

I use following statement to indicate this field before saving a record:

oEditText = oMatrix.Columns.Item("U_UDF").Cells.Item(i).Specific

An there is an error pop-up,

"Unable to cast COM object of type 'System.___ComObject' to interface type 'SAPbouiCOM.EditText ..........."

I have tried using ComboBoxColumn and get the similar error.

Any ideas?

Thanks