       topic_chapter_section = TocStore.getChapterSection(topic_id)
       _.reject(exercise_ids, (exercise_id) ->
         exercise = cache[exercise_id]
+        return true unless cache
         {section} = getImportantTags(exercise.tags)
         section.toString() is topic_chapter_section.toString()
       )

      @_exerciseCache[exercise_id]

    getTeksString: (exercise_id) ->
      tags = @_exerciseCache[exercise_id].tags
      teksTags = _.where(tags, {type: EXERCISE_TAGS.TEKS})
      _.map(teksTags, (tag) ->
        tag.name?.replace(/[()]/g, '')
      ).join(" / ")

    getContent: (exercise_id) ->
      @_exerciseCache[exercise_id].content.questions[0].stem_html

    getTagContent: (tag) ->
      content = getTagName(tag) or tag.id

ExerciseConfig =
  _exercises: []
  _asyncStatus: null
  _exerciseCache: {}
  _unsavedExclusions: {}

  FAILED: -> console.error('BUG: could not load exercises')
