Passcode being unnecessarily offered

When trying to use AirPlay with my iMac that had been previously working with Apple TV (tvOS 9.2.2), a Passcode has started to appear on the TV display when I try to shift the iMac display to Apple TV. There is no corresponding pop-up box on the Mac to input said code, so I am unable to pair the two.

I have rebooted the Mac and have reset the Apple TV and checked to ensure that there is no passcode requirement in settings. The Apple TV works with both my iPhone and iPad without code requirement. Anyone have any ideas? Thanks in advance.