docker-compose sample does not work

Creating _mongo_1 ... error

ERROR: for _mongo_1  Cannot create container for service mongo: b'Invalid container name (_mongo_1), only [a-zA-Z0-9][a-zA-Z0-9_.-] are allowed'

ERROR: for mongo  Cannot create container for service mongo: b'Invalid container name (_mongo_1), only [a-zA-Z0-9][a-zA-Z0-9_.-] are allowed'
ERROR: Encountered errors while bringing up the project.

app:
  image: sdelements/lets-chat
  links:
    - mongo
  ports:
    - 8080:8006
    - 5222:5222

mongo:
  image: mongo:latest