Node callback args not able to assert instanceof Error

I'm making a few simple functions for oauth, one of which validates that there is a scope, redirectUri, and clientId in its options hash. Additionally, if these don't exist, then it should return callback(new Error(...)):

function(opts, callback) {
  var scope = opts.scope;
  var clientId = opts.clientId;
  var redirectUri = opts.redirectUri;

  if (!scope) {
    return callback(new Error('No scope provided'));
  }

  ...
}

In my tests, I test this by asserting that if no scope is provided, that it should return something of type Error and the error should match that description:

it('should require a scope', function() {
  var callback = sinon.spy();
  myOauthFunction({}, callback);

  callback.args[0].should.be.an.Error;
  callback.args[0].toString().should.eql('Error: No scope provided');
});

However, when I run the tests, I get this error:

AssertionError: expected [ [Error: No scope provided] ] to be an error. The second assertion doesn't cause issues, meaning that the argument has a toString property, but isn't of type Error. I'm kind of confused - why isn't it able to assert it's an Error?