Docker complains that bind source path doesn't exist, but it DOES

I'm trying to start a mysql container with a host directory mounted. I'm running a single-node swarm in Docker for AWS. But I can't mount a directory...

This is the section of the docker-compose file I'm using:

mysql:
    image: mysql-custom
    volumes:
        - /mysql:/var/lib/mysql
    ports:
        - "3307:3306"

And this is the launch command I'm trying:

docker stack deploy --compose-file docker-compose.yml stack12

When I launch however, docker swarm rejects the image, saying "invalid mount config for type \"bind\": bind source path does not exist". However, /mysql is a valid directory, I can cd to it and everything, so why is docker saying it does not exist?