Is it possible to serialize interactivity styles within a diagram?

Here's my workflow: I load a diagram from disk. Find a shape I want to make interactive and set its InteractivityStyle:
shape.Style.InteractivityStyle = new NInteractivityStyle(true, null, null, CursorType.Default, "http://my.url.com");

Then I attempt to save the NDrawingDocument in XML format, but the XML serialization stops after writing out only a few lines of XML. See attachment for the output file.

I tried to manually edit the NDX file to add InteractivityStyle tags to the desired shapes. But when I did that, I was no longer able to load the diagram from the file. There were no error messages or exceptions. The following code just returned null:

NPersistencyManager mgr = new NPersistencyManager();
NDrawingDocument document = mgr.LoadDrawingFromFile("diagram.ndx");

What am I doing wrong here? Thanks.