angularjs service is not a function

Question

I have a service like so:

app.service('Utilities', function() {
  this.sum = function(items, prop) {
    var count, total;
    total = 0;
    count = 0;
    if (items === null) {
      total = 0;
    }
    while (count < items.length) {
      total += items[count][prop] * 1 || 0;
      count++;
    }
    return total;
  };
});

and a controller like so:

app.controller('writeCtrl', function($scope, Utilities, students) {
  $scope.students = students;
  $scope.total_age = Utilities.sum($scope.students, 'age');
});

And I keep getting the error

Typerror: Utilities.sum is not a function

Which is confusing because about a dozen other functions under the Utilities service is working fine. What's causing the issue, and how do I get the function to work?

Edit Actual Coffeescript version

app.service 'Utilities', ->
  @sum = (items, prop) ->
    total = 0
    count = 0
    if items == null
      total = 0
    while count < items.length
      total += (items[count][prop]*1 || 0)
      count++
    return total

app.controller 'writeCtrl', ($scope, Utilities, students) ->
  $scope.students = students
  $scope.total_age = Utilities.sum($scope.students, 'age')

Solution:

Coffeescript functions need a return:

App.service 'Utilities', -> 
  .....
  return

Answer 1

Service never returns an object, basically it binds a method or variable to its context; nothing but this & then it returns a new object which contains all the things which were binded to this.

Code

app.service('Utilities', function() {
  this.sum = function(items, prop) {
    var count, total;
    total = 0;
    count = 0;
    if (items === null) {
      total = 0;
    }
    while (count < items.length) {
      total += items[count][prop] * 1 || 0;
      count++;
    }
    return total;
  };

  //
  // ..other utility method should lies here..
  //..do your stuff
});

Update

You should change your coffee script service from

app.service 'Utilities' ->

to

app.service 'Utilities' () ->