Set a custom duration field using VBA throws the message "Invalid duration" when reopening the Project

1. With a custom code we set Duration9 in Project 2013.
2. After executing the code everything seems to be ok.
3. After saving and reopening the Project it shows a message "Invalid Duration", and the value is shown as estimated (Questionmark).

1. In VBA create following code:
   

Sub SetDuration()

    Dim newVal As Long

    newVal = 14400

    Tasks(1).Duration9 = newVal

End Sub

1. Execute Code.
2. Close VBA.
3. Open the Project and you will get the same message box
4. The only way as far as I found out is to manually retype the Duration in that field. What is not exactly what I want.

Is this a bug in MSP or a hidden Feature? Does anyone has an idea?

Best Regards Steffen