Why my script isn't work?

Why my script isn't work? I get error:

Assets/Scripts/cel.js(63,27): BCE0004: Ambiguous reference 'Label': UnityEngine.GUI.Label(UnityEngine.Rect, UnityEngine.GUIContent), UnityEngine.GUI.Label(UnityEngine.Rect, UnityEngine.Texture), UnityEngine.GUI.Label(UnityEngine.Rect, String).

It's a scripit:

 #pragma strict
 static var al;
 static var m1 = "Mission: overheard a conversation between two men in suits";
 static var m2 = "Mission: ---";
 static var m3 = "Mission: ---";
 static var m4 = "Mission: ---";
 static var m5 = "Mission: ---";
 static var m6 = "Mission: ---";
 static var m7 = "Mission: ---";
 static var m8 = "Mission: ---";
 static var m9 = "Mission: ---";
 static var m10 = "Mission: ---";
 static var m11 = "Mission: ---";
 
 var i: boolean;
 function Start () {
     switch (Application.loadedLevelName) {
         case m1:
             al = m1;
             break;
         case m2:
             al = m2;
             break;
         case m3:
             al = m3;
             break;
         case m4:
             al = m4;
             break;
         case m5:
             al = m5;
             break;
         case m6:
             al = m6;
             break;
         case m7:
             al = m7;
             break;
         case m8:
             al = m8;
             break;
         case m9:
             al = m9;
             break;
         case m10:
             al = m10;
             break;
         case m11:
             al = m11;
             break;
     }
 }
 function Update () {
     if (Input.GetKey (KeyCode.I)) {
         i = true;
     } else {
         i = false;
     }
 }
 
 function OnGUI () {
     if (i) {
         GUI.Label (Rect (5, 5, 200, 40), al);
       }
 }