Error -1029 by Update a UDF

Hi,

We have a AddOn that only use the DIAPI. It must update a UDF on A/R Invoice but the update will not work.

I became the error -1029

Here a cut from the code

If (vDoc.GetByKey(oRecordSet.Fields.Item(1).Value) = True) Then

vDoc.UserFields.Fields.Item("U_saMahn").Value = oRecordSet.Fields.Item(0).Value

vDoc.Update()

Have anyone a idea?