Internal display can't show film. This film can only be played on displays that support HDCP

Hi,

This year I bought an iMac and I've connected it to the Internet and I am trying to some films I recently bought in iTunes Store, to play first on it's own internal display. I would also like to be able to cast it to a big TV for ease of watching as a family and to make use of the amazing Hue Sync.

I have bought a number of films lately, but when I go to play them on my iMac I keep getting the error message "This film can only be played on displays that support HDCP (High-bandwidth Digital Content Protection).".

Does anyone know how I can get this working?