I tried to add validation for normlizeDiscountCode method, but it kept said that it couldn't pass the test

public class Order {
  private String itemName;
  private int priceInCents;
  private String discountCode;

  public Order(String itemName, int priceInCents) {
    this.itemName = itemName;
    this.priceInCents = priceInCents;
  }

  public void applyDiscountCode(String discountCode) {

    this.discountCode = normalizeDiscountCode(discountCode);
  }
  private String normalizeDiscountCode(String discountCode){
    this.discountCode = discountCode;    
     for (char letter: discountCode.toCharArray()){       
      if( !Character.isLetter(letter) || letter != '$'){       
        throw new IllegalArgumentException("Invalid discount code");
      }
     }
    return discountCode.toUpperCase(); 
  }
}