DoInstead for private method through _Accessor class

Is there any issue trying to control the behavior of a method that is being accessed through the "Create Private Accessor" option in visual studio?

I get...
System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index.

Stack trace is on
System.System.Collections.ArrayList.get_Item(Int32 index)
after some callls through isolator.

If I change the methods to other than private, and use the actual class instead of the accessor class the same code runs fine.

If it matters, this is a "live" object, not a fake. (though I'm using DoInstead to intercept behavior).

thanks :)