C# Object referance not set to an instance of an object

Hello! i would like help with a problem i am currently having. The problem is:
NullReferenceException: Object reference not set to an instance of an object
PlanePilot.Update () (at Assets/Standard Assets/PlanePilot.cs:15) And the script is
1: usingUnityEngine;
2 usingSystem.Collections;
3
4 publicclassPlanePilot : MonoBehaviour {
5 publicfloatspeed = 90.0f ;
6
7 //Usethisforinitialization
8 voidStart () {
9 Debug.Log ("plane pilot script added to: "...

C# Object referance not set to an instance of an object