Docker compose error "In file './docker-compose.yml', service 'punjab' must be a mapping not a string."

I am getting a strange error in my docker-compose.yml file.

I have prepared a docker-compose file for the stack punjab connection manager, ejabberd and mysql.

Below is the docker-compose.yml file

version: '2'
services:
  punjab:
    image:punjab
    ports
     - 5280:5280
    links
     - ejabbberd:ejabberd
  ejabberd:
    image: ejabberd
    depends-on:
      - mysql
    links:
      - mysql:mysql
  mysql:
    image:mysql

When I run the command docker-compose up from the command line and from the same directory where I have the docker-compose.yml file, I get the following error.

ERROR: In file './docker-compose.yml', service 'punjab' must be a mapping not a string.

I parsed the yml file using yamllint as well and the file is correctly formatted.

Although in theory the file format may look correct, there are some things that are wrong.

They are:

1. Use depends_on instead of depends-on because that is the correct syntax.

2. For best practice, use a blank space before informing the image like: image: punjab.

3. On ports and links at punjab service, you forget the colon :

4. The images ejabberd and punjab do not exist on Docker Hub so they must exist in your local repository.

This is an example to use a docker-compose.yml:

version: '2'
services:
  punjab:
    image: punjab
    ports:
      - "5280:5280"
    links:
      - ejabberd

  ejabberd:
    image: ejabberd
    depends_on:
      - mysql
    links:
      - mysql

  mysql:
    image: mysql