Coffescript, undefined is not a function in Ember.js controller when calling internal function

My controller looks like this:

App.ExaminationController = Ember.ObjectController.extend

  amountErrors: null

  actions:
    saveChanges: ->
      if @get('model.isDirty')
        @get('model').save().then(
          ->
          (errors) ->
            @setAmountErrors errors
        )

    cancelChanges: ->
      @get('model').rollback()

  submitValue: ( ->
    if @get('isDirty')
      'Zapisz'
    else
      'OK'
  ).property('isDirty')

  setAmountErrors: (errors) ->
    if errors.errors.amount
      @amountErrors = errors.errors.amount[0]a
      console.log @amountErrors

When the line @setAmountErrors errors is executed i get TypeError: undefined is not a function And although i tried and validated my code i looks like it's syntax is ok to me. I tried to modify it in all the possible ways of which i could think of. And when I drop the function call and write function call instead the function call like this:

App.ExaminationController = Ember.ObjectController.extend

  amountErrors: null

  actions:
    saveChanges: ->
      if @get('model.isDirty')
        @get('model').save().then(
          ->
          (errors) ->
            if errors.errors.amount
              @amountErrors = errors.errors.amount[0]
              console.log @amountErrors
        )

    cancelChanges: ->
      @get('model').rollback()

  submitValue: ( ->
    if @get('isDirty')
      'Zapisz'
    else
      'OK'
  ).property('isDirty')

It suddenly started to work like intended. But i want to call a function and not write away body of this function. How to do it? What am I missing? Does it has to do anything with my coffescript code, or am I missing something in the Ember controller itself?