Where is this (-1) coming from in my change level script?

I have a script that changes from Level 1-1 to level 1-2 just fine.

    function OnTriggerEnter (other : Collider){
       if(other.gameObject.tag == "Player"){
          Application.LoadLevel("1-2");
          }
          }

But I have a script that is exactly the same but to loads level 1-3 instead.

    function OnTriggerEnter (other : Collider){
       if(other.gameObject.tag == "Player"){
          Application.LoadLevel("1-3");
          }
          }

But when I enter the trigger I get this error:

Level '1-3' (-1) couldn't be loaded because it has not been added to the build settings. To add a level to the build settings use the menu File->Build Settings... UnityEngine.Application:LoadLevel(String) Level 1-3:OnTriggerEnter(Collider) (at Assets/Scripts/NextLevels/Level 1-3.js:5)

Where is the (-1) Coming from? I have level 1-3 in the Scenes in build menu. The same thing was happening when I was trying to go from 1-1 to 1-2 but it just stopped randomly.

Thanks in advance for any help.