Developer Console problems..... (in-game console)

So, I'm trying to make an in-game developer console and I've ran into a problem... Can't really understad the error.... :P

Error :

 ArgumentOutOfRangeException: startIndex + length > this.length
 Parameter name: length
 System.String.Substring (Int32 startIndex, Int32 length) (at /Users/builduser/buildslave/mono-runtime-and-classlibs/build/mcs/class/corlib/System/String.cs:356)

And my code looks like this :

 using UnityEngine;
 using System.Collections;
 
 public class DevConsole : MonoBehaviour {
 
     public string devConsole;
 
     void OnGUI() {
 
         devConsole = GUI.TextField (new Rect (10, 100, 400, 30), devConsole);
         if (Input.GetKeyUp (KeyCode.Return)) {
 
             Debug.Log ("Return was pressed");
 
             if (devConsole.Length > 4 && devConsole.Substring(0, 4) == "say ") {
                 Debug.Log (devConsole.Substring(4, devConsole.Length));
                 devConsole = "";
             }
 
         }
 
     }
 }