HP8710 won't print greyscale without cyan cartridge

Hi, any advice will be welcomed ....
The yellow cartridge supplied with my Officejet Pro8710 ran out. I got a screen message to say the ink was low and an option to print greyscale only. That was great and kept me going while I ordered a new yellow cartridge.
I just fitted the (genuine HP) yellow cartridge and immediately got a message saying the cyan cartridge was empty and I cannot print at all until it is replaced. So I was actually better off before I fitted the new cartridge.
I don't...

HP8710 won't print greyscale without cyan cartridge