Using diagnostic mode on 27 inch iMac

I was trying to get my 27 inch core i7 iMac to boot into diagnostic mode this morning by holding down the 'D' key during bootup. It just wouldn't work and kept booting up normally. Then it occurred to me that maybe this is due to the fact that this iMac comes with a bluetooth keyboard. I happened to have a usb apple keyboard around so I plugged that in, rebooted while holding down the 'D' key and voila, it worked.

Am posting this in case anyone else is trying to go into diagnostic mode and can't figure out why it isn't working.