How to access Twitter's OpenAPI?

I'm trying to access Twitter's OpenAPI, which I believe is found here: https://api.twitter.com/labs/1/openapi.json

When I do this, I keep getting error 131:

• Is this the correct uri to access it?
• Am I maybe missing something, like a querystring param?

BTW, Error 131 is an "internal" version of HTTP Error 500.