InvalidCastException with my GUI Textures

Hey all,

I'm trying to make a basic results screen that pops up at the end of my stage. The basic gist is that my game object (housing only a GUITexture component and the script listed below) accesses my Performance script to find out how many lives the player has left. I then use a switch statement to find the appropriate GUITexture which I will then fade in after the results have been posted.

Problem is that I keep getting an "InvalidCastException: Cannot cast from source type to destination type" error. Any ideas on what I'm doing wrong? (Fade-in code not included yet.)

 using UnityEngine;
 using System.Collections;
 
 public class Rank : MonoBehaviour {
 
     public GUITexture playerRank;
     
     Performance performance;
     
     GameObject manager;
     
     // Use this for initialization
     void Start () 
     {
         manager = GameObject.FindGameObjectWithTag("Manager");
         performance = manager.GetComponent<Performance>();
     
     }
     
     // Update is called once per frame
     void Update () 
     {
         
     }
     
     public void RankCheck()
     {
         switch (performance.lifeCount)
         {
             case 1:
                 playerRank = (GUITexture)Resources.Load("grade_C");
                 break;
             
             case 2:
                 playerRank = (GUITexture)Resources.Load("grade_C");
                 break;
             
             case 3:
                 playerRank = (GUITexture)Resources.Load("grade_B");
                 break;
             
             case 4:
                 playerRank = (GUITexture)Resources.Load("grade_B");
                 break;
             
             case 5:
                 playerRank = (GUITexture)Resources.Load("grade_A");
                 break;
         }
     }
 }

Thanks for your time!

BKF