Grooveshark OAuth authentication issue

I am trying to use the Grooveshark Oauth api to get an access token. The official documentation says

Lightweight OAuth

We've implemented lightweight OAuth integration for public APIs. Start by directing users to https://auth.grooveshark.com/ with app GET and callback parameters. The app value must be equal to your API key. The callback should be a URL that the user can be redirected to upon successful authentication, or the user to de-authenticate. One of two possible GET parameters will be enabled depending on whether the authentication was successful: token when the token was generated successfully and canceled when the user cancels.

I tried using the following url https://auth.grooveshark.com?app=API_KEY&callback=http://www.someurl.com but I keep getting the following error

Allow access to your account? The website is requesting access to your Grooveshark account. However, there was a problem with the request. Try again.

Any clue that I might be wrong when shaping the OAuth URL?