Reverse animation loop stops at frame zero

Im trying to do a simple animation with my 2d sprite.

I got 20 frames in one animation state. Through a script Im controlling the speed of the animation. Hitting the right arrow Im setting the speed to 1 and the animation is looping as wanted. When Im hitting the left arrow, I set the speed to -1, so it's going reverse but it's stopping at key frame 0 and not looping back. Why?

I was trying to access to "anim.animation" but it raises an error MissingComponentException: There is no animation attached to the game object.

Anyone can help me?

 using UnityEngine;
 using System.Collections;
 
 public class MyScript : MonoBehaviour {
 
     public float speed = 1.0f;
     public string axisName = "Horizontal";
     public Animator anim;
 
     // Use this for initialization
     void Start () {
         anim = gameObject.GetComponent<Animator> ();
     }
     
     // Update is called once per frame
     void Update () {
         if(Input.GetAxis(axisName)<0){
             anim.speed = -1;
         }else if(Input.GetAxis(axisName)>0){
             anim.speed = 1;
         }
         else{
             anim.speed = 0;
         }
     }
 }