transform.position assign attempt for not valid. Input position is { NaN, NaN, NaN }

On my first project, I get this error .... help please.

using UnityEngine;

using System.Collections;

public class motionControl : MonoBehaviour {

 private Animator animator;
 private float speed=0.0f;

 void Start () {
     animator = GetComponent<Animator> ();
 }
 
 void Update () {
     if (animator) {
         speed=0.0f;
         if(Input.GetKey ("w"))
             speed=1.0f;
             }
     animator.SetFloat ("Speed", speed);
 }

}