If you're running the example on the website/in a browser, it's probably CORS stopping you.

Try using a backend language or just curl and it should be fine.

Well no, because my browser isn't doing the request. The underlying Node app (the Jam API) does it.

I found it: The API responds with an HTTP 500 error if you use CSS selectors that don't select anything or are simply invalid.

Probably makes sense to add some Exception handling right there.

I had been trying to figure out what would be causing this issue, thanks for pointing it out, I've pushed a fix real quick that will respond whether JSON is invalid or a CSS selector wasn't found on the provided URL.

I'm also getting 500s. Looks to be a CORS issue. I've tried about a dozen big name sites so far without any luck.

Nonetheless, a great idea!

Edit: Just tried it in Node and it works brilliantly. Cool project.

A CORS restriction perhaps?