Error parsing your wercker.yml: yaml: line 1: mapping values are not allowed in this context

Symptom:

   When we create a wercker.yml in github to build a CI/CD pipeline via app.wercker.com
    It error out :
 Error parsing your wercker.yml: yaml: line 1: mapping values are not allowed in this context

   The wercker.yml file is :
box: python:  2.7
build:
   steps:
      - internal/docker-build:
        dockerfile: Postgres-Dockerfile
        image-name: PostgresImage
        build-args: $POSTGRESS_BUILD_ARGES

Solution:

   This is yaml syntax error , nothing related to wercker . The error is from space between python:  and 2.7
   Remove space and correct one is  python:2.7