Help with strings

Hello guys I'm running into an issue. I have a string that I want to lose a letter if backspace is pressed. Here is my code.
if(Input.GetKeyDown (KeyCode.Backspace)){
stringToEdit= stringToEdit.Substring(0,stringToEdit.Length-1);
Debug.Log ("Backspace is pressed");
}
This should work and I know it has something to do with the stringToEdit.Lenght. If i put in my own value for the length it would work just fine. Does anyone know why this is happening?
Incase you need to see the entire code here...

Help with strings