diff -r fa9dfbfc4145 -r b7cc9d09aefe Hardware/HDD/HDD.cs
--- a/Hardware/HDD/HDD.cs	Thu Aug 12 20:53:27 2010 +0000
+++ b/Hardware/HDD/HDD.cs	Sun Aug 15 14:46:58 2010 +0000
@@ -117,8 +117,9 @@
     #pragma warning restore 67    
 
     public void Accept(IVisitor visitor) {
-      if (visitor != null)
-        visitor.VisitHardware(this);
+      if (visitor == null)
+        throw new ArgumentNullException("visitor");
+      visitor.VisitHardware(this);
     }
 
     public void Traverse(IVisitor visitor) { }