Karma/Jasmine/Angular-Mocks "not a function got string"

I am running some unit tests based off of this website: http://ift.tt/1S5NWIw

I've wrote an app.js file and a MathController.js file.

Enclosed is the relevant code:

app.js

(function(){
    var app = angular.module('app', ['ui-router', 'ng-resource', 'ui-bootstrap']);
   // app.config stuff here
}());

MathController.js

(function(){
    angular.module('app').controller('MathController', ['$scope', function ($scope){

        $scope.sum = function(){
            $scope.z = $scope.x + $scope.y;
        };

    }]);
}());

And here is the test spec file:

MathController.spec.js

describe('MathController', function(){

    beforeEach(angular.mock.inject('app'));
    var $controller;

    beforeEach(angular.mock.inject(function(_$controller_){
        $controller = _$controller_;    
    }));


    it('should equal 3', function(){
        var $scope = {};
        var controller = $controller('MathController', {$scope: $scope });
        $scope.x = 1;
        $scope.y = 2;
        $scope.sum();
        expect($scope.z).toBe(3);
    });

});

And this is the error I get in the console:

PhantomJS 2.1.1 (Linux 0.0.0) MathController should equal 3 FAILED
    Error: [ng:areq] http://ift.tt/1WxbQ3U (line 22)

.....some extra stuff omitted
    workFn@/root/lib/angular/angular-mocks.js:2401:26
    undefined
    Error: [ng:areq] http://ift.tt/1sMPzCh in /home/root/lib/angular/angular.min.js (line 22)
.....some extra stuff omitted
    /home/root/spec/MathController.spec.js:13:31
PhantomJS 2.1.1 (Linux 0.0.0): Executed 1 of 1 (1 FAILED) ERROR (0.041 secs / 0.006 secs)

So it says MathController is not a function, and it says angular picked up a string when it should have picked up a function. Can anyone point me in the right direction of how to fix this?