Ember reopen don't let me access to atributes

I'm new to Ember and I'm writing my first app. I'm working with version 1.13.13.

What I'm trying to do is to override a function from a class that I comes from version 0.2.1 of a package I installed (http://ift.tt/2aGz7ea).

Here is the code I'm using:

`import Ember from 'ember'`
`import MultiselectCheckboxOptionComponent from
  'ember-multiselect-checkboxes/components/multiselect-checkbox-option'`

multiselectCheckboxesPatch =
  name: 'ember-multiselect-checkboxes-patch'
  initialize: ->
    MultiselectCheckboxOptionComponent.reopen
      isSelected: Ember.computed 'value', 'selection', (_, checked) ->
        if arguments.length > 1
          selected = this.get('selection').contains(this.get('value'))

          if checked and not selected
            this.get('selection').addObject(this.get('value'))
          else if not checked and selected
            this.get('selection').removeObject(this.get('value'))

          # Added this line to fix variant of http://ift.tt/2aSDm9V
          @set('selection', Ember.copy(this.get('selection')))

        this.get('selection')?.contains(this.get('value'))

`export default multiselectCheckboxesPatch`

But the issue I'm facing is that this.get('selection') seems to be undefined. I don't understand why since selection is an attribute from the class that I'm reopening. Also, I think this code used to work in older versions of ember since I got it from another project.

Thank you!