state `apt.update` can't run standalone

[daks]

I thought I'd be able to use this apt.update state to update the apt db on servers, so I tried

salt 'minion' state.sls apt.update

It fails with

          ID: apt-get update
    Function: cmd.wait
      Result: False
     Comment: The following requisites were not found:
                                 watch:
                                     file: /etc/apt/sources.list
                                     file: /etc/apt/sources.list.d
     Started: 13:42:21.398096
    Duration: 0.061 ms
     Changes:   

README lists this state so it looks like it's callable standalone (not just for internal use) but it seems not.