Ember renders html via link-to, but json via typing url in browser

This doesn't happen locally in development, but after deploying to Heroku, if I navigate to my app's root page it loads fine and if I click the "posts" link it loads all my posts in html. But if I navigate to the same page by typing the url in the browser's url input, or even if I simply click the refresh button, it loads json instead of html. Inspecting the request, I see that it sends no query params (so the json is not paginated) and there is no Content-Type in the request headers.

Hoping this is a telltale sign of some obvious problem. If not, I could provide more info about my ember/rails setup.