Alan that is the reason it works when they are taped because it isn't reporting the combined aperture. You shouldn't need to have it taped on the 300 though since even with the TC you are still just at 5.6 and normally only cameras that wont AF with F\8 as a starting aperture needs to have the pins taped. I use a 1.4TC all the time on mine without any problems. The way I understand it would only need to be done on F\5.6 lenses and above.