TypeError: $ionicAuth.signup is not a function

Question

I am using $ionicAuth service that deals with registration and logging in/out. I followed this tutorial https://docs.ionic.io/services/auth/#emailpassword-authentication and https://docs.ionic.io/setup.html - for setting up ionic cloud and using the services. When I try to register, I get this error - $ionicAuth.signup is not a function.Please help me to fix this.

$scope.register = function(){

        $scope.error = '';
        $ionicAuth.signup($scope.registerData).then(function() {
            // `$ionicUser` is now registered
            $ionicAuth.login('basic', $scope.registerData).then(function(){
              $state.go('login');
            });
        }, function(err) {

            var error_lookup = {
                'required_email': 'Missing email field',
                'required_password': 'Missing password field',
                'conflict_email': 'A user has already signed up with that email',
                'conflict_username': 'A user has already signed up with that username',
                'invalid_email': 'The email did not pass validation'
            }

            $scope.error = error_lookup[err.details[0]];
        })
    }