AutoCAD Inventor :: Modifying Custom IProperties In C#

I am using some code from the June 2010 "Mod the machine" posted by Brian Erkins.I keep getting an error in C# (express 2010), Please see attached screen dump.

I don't see any obvious difference in my code when compared to Brian's (in fact at one point I copied and pasted his code and still had the same error). Here is the

private void button1_Click(object sender, EventArgs e)        
{            
Inventor.Application invApp = null;            
invApp = (Inventor.Application)System.Runtime.InteropServices.Marshal.GetActiveObject("Inventor.Application");
Inventor.Document doc = invApp.ActiveDocument;            
Inventor.PropertySet customPropSet;            
customPropSet = doc.PropertySets["Inventor User Defined Properties"];