Connection to MS-Project via AutomationFactory

I'm developing an ERP-Solution based on the Silverlight RAD tool LightSwitch.

When I try to connect to MS-Project via AutomationFactory an want to add a TaskDependency:

project.ActiveProject.Tasks(2).TaskDependencies.Add(project.ActiveProject.Tasks(1), 1, 1)

I get an error statemement.

Ausnahme von HRESULT: 0x800A044D
Ungültiger Argumentenwert.
(HelpFile=C:\Programme\Microsoft Office\Office12\VBAPJ.CHM#131074)

Does someone knows the right data type or data format that has to be used?

Many thanks in advance