Validation failed in file './docker-compose.yml'

i am experiencing wired issue, when i run docker-compose up , got error message “Validation failed in file ‘./docker-compose.yml’, reason(s):
web.links contains an invalid type, it should be an array”.
anybody knows the reason ?

here is my docker-compose.yml file

web:
image: lambkin/foodtrucks-web
command: python app.py
ports:
- "5000:5000"
volumes:
- .:/code
links:
- es
es:
image: elasticsearch

thanks.