I'm using the following code:

using UnityEngine;
using System.Collections;

public class TestScript{

 private GUISkin custom_skin;
 public bool guiEnabled;

 public TestScript(GUISkin new_skin){
 custom_skin = new_skin;
 }

 public void OnGUI(){
 if(guiEnabled){
 GUI.skin = custom_skin;
 GUI.Label(new Rect(0,0,50,50),"Blah");
 }
 }
}

The problem is that I'm also getting the following error: ArgumentException: You can only call GUI functions from inside OnGUI.

It works fine in another script that is running in the same scene, but for some reason this one won't. I looked at the other script, and it's all set up in exactly the same way, but for some reason I'm getting this error all of a sudden.