COMException 8002008 Bad variable type

I'm not too familiar with COM etc but we have an app that calls an
IDispatch scriptable application.  The application has been updated such
that a method that used to be defined as having a param of System.Object
is now defined as having System.String.  All the calls to methods with
System.Object still work fine with a java.lang.String and a VT_BSTR is
printed to the screen, the call to the only System.String fails with the
8002008 exception.  Any ideas on what I have to change to make this work?

The COM method sig is:
    public abstract new void ExportAsPDF (System.String filename)

I hope this an easy newbie type problem, the JAWIN project has made it
relatively easy to integrate third party apps into our java project.

Thank you