I've looked around and can't seem to find a solution to this error, I don't seem to be passing a string instead of a function anywhere, could someone take a look and see what I'm doing wrong? Thanks:
register.controller.js
(function () {
    'use strict';
    angular
      .module('membership.authentication.controllers')
      //register the controller
      .controller('RegisterController', RegisterController);
    RegisterController.$inject = ['$location', '$scope', 'Authentication'];
    //'Authentication' is the function from the authentication service
    function RegisterController($location, $scope, Authentication) {
        var vm = this;
        vm.register = register; // allows the access of the function register()
        function register(){
            // this is calling the register method of the Authentication service
            Authentication.register(vm.email, vm.password, vm.username);
        }
    }
})();
authentication.service.js
(function () {
    'use strict';
    angular
    // where the service is located
        .module('membership.authentication.services')
        .factory('Authentication', Authentication) // register the factory
        //'Authentication' is the name of the factory, also the name of the function
    Authentication.$inject = ['$cookies', '$http'];
    // define the factory that was registered, using dependencies
    function Authentication($cookies, $http) {
        var Authentication = {
            register: register // define service as named object, more readable
        };
        return Authentication;
        function register(email, password, username) {
            return $http.post('/api/v1/accounts/', {
                username: username,
                password: password,
                email: email
            });
        }
        }
})();
membership.config.js
(function () {
    'use strict';
    angular
      .module('membership.config', [])
      .config(config);
    config.$inject = ['$locationProvider'];
    function config($locationProvider){
        $locationProvider.html5Mode(true);
        $locationProvider.hashPrefix('!');
    }
})();
membership.js
(function () {
    'use strict';
    angular
      .module('membership', [
        'membership.authentication',
        'membership.routes',
        'membership.config'
        ]);
    angular
      .module('membership.routes', ['ngRoute']);
    angular
      .module('membership')
      .run('run')
    run.$inject = ['$http']
    function run($http) {
        $http.defaults.xsrfHeadName = 'X-CSRFToken';
        $http.defaults.xsrfCookieName = 'csrftoken';
    }
})();
membership.routing.js
(function () {
    'use strict'
    angular
      .module('membership.routing', [])
      .config(config);
    //$routeProvider adds routing to the client
    config.$inject = ['$routeProvider'];
    function config($routeProvider) {
        //when() takes two arguments: a path and an options object
        $routeProvider.when('/register', {
            controller: 'RegisterController',
            controllerAs: 'vm',
            templateUrl: '/static/templates/authentication/register.html'
        }).otherwise('/');
    }
})();
edit:
index.html
<!DOCTYPE html>
<html ng-app="membership">
<head>
  <title>thinkster-django-angular-boilerplate</title>
  <base href="/" />
  {% include 'stylesheets.html' %}
</head>
<body>
  {% include 'navbar.html' %}
  <div class="container-fluid">
    <div class="row">
      <div class="col-xs-12 ng-view"></div>
    </div>
  </div>
  {% include 'javascript.html' %}
</body>
</html>
The angularjs line that has 'fn':
 function anonFn(fn) {
  // For anonymous functions, showing at the very least the function signature can help in
  // debugging.
  var args = extractArgs(fn);
  if (args) {
    return 'function(' + (args[1] || '').replace(/[\s\r\n]+/, ' ') + ')';
  }
  return 'fn';
}
function annotate(fn, strictDi, name) {
  var $inject,
      argDecl,
      last;
  if (typeof fn === 'function') {
    if (!($inject = fn.$inject)) {
      $inject = [];
      if (fn.length) {
        if (strictDi) {
          if (!isString(name) || !name) {
            name = fn.name || anonFn(fn);
          }
          throw $injectorMinErr('strictdi',
            '{0} is not using explicit annotation and cannot be invoked in strict mode', name);
        }
        argDecl = extractArgs(fn);
        forEach(argDecl[1].split(FN_ARG_SPLIT), function(arg) {
          arg.replace(FN_ARG, function(all, underscore, name) {
            $inject.push(name);
          });
        });
      }
      fn.$inject = $inject;
    }
  } else if (isArray(fn)) {
    last = fn.length - 1;
    assertArgFn(fn[last], 'fn');
    $inject = fn.slice(0, last);
  } else {
    assertArgFn(fn, 'fn', true);
  }
  return $inject;
}
I have been following this tutorial and I can't seem to find any differences, other than the fact that the author didn't include an empty array if when not giving dependencies. Which I did
