Inserting an IInterface element to a Class Diagram

Plug-in & Open API
1

I am attempting to add an IInterface element to a class diagram using the following statement:

diagElem = diagramMgr.createDiagramElement ( diag, interfaceElement);

where diagElem is of type DiagramElement, diagramMgr is an instance of DiagramManager, diag is a IClassDiagramUIModel, and interfaceElement is an IInterface.

I get the following message in the vp.log file:

[Fri Sep 09 14:58:31 PDT 2011] [error] Class not found for ShapeType: Interface
[Fri Sep 09 14:58:31 PDT 2011] [message] Exception in thread "Thread-13"
[Fri Sep 09 14:58:31 PDT 2011] [message] java.lang.NullPointerException
[Fri Sep 09 14:58:31 PDT 2011] [message] at v.bwgw.a(:184)
[Fri Sep 09 14:58:31 PDT 2011] [message] at v.bwgw.createDiagramElement(:106)

What is the correct method for inserting an IInterface element into a class diagram? I am using VP Suite 5.2.