NewExplorer Event in a C# add-in

I'm writing an add-in in C# for Outlook 2003 (SP1) and using the default PIA.
I’m getting a NullReferenceException with the message "Object reference not
set to an instance of an object.” when running the following code:

applicationObject.Explorers.NewExplorer += new
Microsoft.Office.Interop.Outlook.ExplorersEvents_NewExplorerEventHandler(Explorers_NewExplorer);

The Explorers event is just fine. Any ideas about this?