Why can't I deploy a stage environment to Heroku with TravisCI?

I am trying to deploy a stage environment to Heroku with TravisCI. The Heroku account for stage is on a different account than the production environment. I do not have the production environment set up to deploy to Heroku via TravisCI, because I want to get it working before I overwrite anything. Here is my .travis.yml

language: node_js
node_js:
  - node
  - '7'
before_deploy:
  - npm run build
deploy:
  provider: heroku
  app: 
    stage: morning-harbor-36163
  on: 
    all_branches: true
  api_key:
    stage: ...top secret key

When travis runs, the error I get is:

Preparing deploy
No stash found.
missing api_key
failed to deploy

What am I doing wrong?

via jhamm