ENVY 7640 will not go into single cartridge mode, just says 'Cartridges appear to be missing'

I've used my ENVY 7640 in single cartridge mode before, but this time with only the black cartridge installed, it reports 'Cartridges appear to be missing' instead of automatically switching to single cartridge mode. I can't print anything. When I press OK, the printer cancels the job, a process which, interestingly, never ends and requires a power cycle to get out of.