GitHub ERROR probot: Integration not found, documentation_url

Question

I had issues with authentication when working with GitHub's Probot app. When I ran the app using npm start, I would get

github: Github request: GET /app/installations - 404 Not Found

and

ERROR probot: {"message":"Integration not found", "documentation_url": "https://developer.github.com/v3"}

I later found that the reason I was getting this issue was because I was working from a GitHub Enterprise account and needed to add an extra variable to my .env file:

GHE_HOST=github.your_company.com

After restarting my app, I no longer had those issues.