Why this unit test causes 'JSON.stringify cannot serialize cyclic structures' exception?

I want to use a unit test to check if an element is visible after a certain action (_toggleWidget function in this case).

  it 'should show the bar after toggle', ->
    view = new MyView
    view._toggleWidget

    expect(view.$navBar).to.be.visible

The test failed with this error message: TypeError: JSON.stringify cannot serialize cyclic structures.

Here is MyView classs

  class HeaderView extends Backbone.View

    initialize: ->
      @$el.html template()
      @$navBar   = @$ '.nav-bar'