update udf in UI document lines

I created a Marketing Documents/Rows UDF as Text.

I created a popup form to edit the values for the Text Field, when I finish update and I press the update button and I get the following exception: "Form - Bad value"

example:

Dim oFrm As Form = oApp.Forms.Item("frmID")

Dim oMtx As Matrix = CType(oFrm.Items.Item("38").Specific, Matrix)

CType(oMtx.Columns.Item("U_MyField").Cells.Item(DocLine).Specific, EditText).Value = "test"

Is it even posible in the UI the change the value for an existing SAP object? or is it only in the matrix that exists this problem?

Message was edited by: Yuval Lerner