key to enable/disable

Sounds simple enough. I use a footsteps system ( different footsounds for different materials) that I want to completely turn off for when my character is sneaking ( as opposed to running, jumping walking etc). My script seems to work...somewhat, in that for the most part the footstep sounds stop but not entirely because if when I use the arrow keys to steer the character a footsound is heard. So, I press right shift key to get character to sneak and disable footsounds, releasing the key turns footsound back on. But while the right shift key is down, pressing another key ( arrow keys to steer character) it seems to enable the footsound for that instant I pressed the arrow key down.

 using UnityEngine;
 using System.Collections;
 
 public class TurnOnFootsteps : MonoBehaviour {
     void Update() {
         if (Input.GetKeyUp(KeyCode.RightShift))
             GetComponent<AdvFootstepSystem>().enabled = true;
 
 
 
         if (Input.GetKeyDown(KeyCode.RightShift))
             GetComponent<AdvFootstepSystem>().enabled = false;
     }
 }