Cannot read property setRoot undefine

import {Page, NavController} from ‘ionic-angular’;
import {WorkoutService} from ‘…/…/services/workout.service’;
import {WorkoutsPage} from ‘…/workouts/workouts’;

@Page({
templateUrl: ‘build/pages/add-workouts/add-workouts.html’,
providers: [WorkoutService]
})

export class AddWorkout {
static get parameters(){
return [[NavController], [WorkoutService]];
}
public title;
public note;
public type;
public _workoutService: WorkoutService;
public _workouts;
public nav;

constructor(nav, WorkoutService){

    this._workoutService = WorkoutService;
    this.title;
    this.note;
    this.type;
}

onSubmit(){
   var workout = {
       title: this.title,
       note: this.note,
       type: this.type
   }

   // Add Workout
   this._workoutService.addWorkout(workout).subscribe(workouts => {
       this._workouts = workouts
   },
   err => console.log(err),
   () => console.log('Workout Added'));

     this.nav.setRoot(WorkoutsPage);
}

}