Pass list of integers as parameters to Ember transitionTo()?

I'm trying to pass a list of integers (of the form [1,5]) as params in transitionTo in Ember 3.14. I've verified that the list can be read within this function:

  afterModel() {
    this.transitionTo('newRoute', this.modelFor('lastLevel').list);
  },

But I always get this error:

Cannot read property '0' of undefined TypeError: Cannot read property '0' of undefined

How should I transform the list so it will pass into transitionTo()?