I have found the answer, (not that my lisp function is interactive).
AutoCAD .NET Developer's Guide
Guidelines for Event Handlers
Do not attempt any interactive functions from an event handler.
Attempting to execute interactive functions from within an event handler can cause serious problems, as AutoCAD may still be processing a command at the time the event is triggered. Therefore, you should always avoid requesting for input at the Command prompt, as well as object selection requests, and using the SendStringToExecute method from within event handlers.