As a beginner in close up photography, I am suing extension tubes with my Canon 550D, standard 18-55 mm lens. I notices that when I use this, the f-stop meter in the camera does not work and always shows f00. I am using the Fotodiox ( http://www.amazon.com/Fotodiox-Canon-EOS-Extension-Close-up/dp/B003Y60DZO/ref=sr_1_2?ie=UTF8&qid=1330854192&sr=8-2 ) extension tubes that I bought from Amazon. Is this the normal behavior or am I connecting these extension tubes wrongly ?

thanks

As a beginner in close up photography, I am using extension tubes with my Canon 550D and standard 18-55mm lens. I noticed that when I use this, the f-stop meter in the camera does not work and always shows f00. I am using the Fotodiox extension tubes that I bought from Amazon. Is this normal or am I connecting these extension tubes wrong?