passenger-statusでエラーが出た時

sudo passenger-status

を実行したとき、このようなエラーが出ました。

ERROR: Phusion Passenger doesn't seem to be running. If you are sure that it is running, then the causes of this problem could be:

You customized the instance registry directory using Apache's PassengerInstanceRegistryDir option, Nginx's passenger_instance_registry_dir option, or Phusion Passenger Standalone's --instance-registry-dir command line argument. If so, please set the environment variable PASSENGER_INSTANCE_REGISTRY_DIR to that directory and run passenger-status again.
The instance directory has been removed by an operating system background service. Please set a different instance registry directory using Apache's PassengerInstanceRegistryDir option, Nginx's passenger_instance_registry_dir option, or Phusion Passenger Standalone's --instance-registry-dir command line argument.

他のサイトなどを参考すると、PASSENGER_INSTANCE_REGISTRY_DIRを設定するなどがありましたが、私の場合は恐ろしい凡ミスで、

include /etc/nginx/passenger.conf;

のコメントアウトを忘れていました。