I'm currently attempting to create a Bulls and Cows game for a school mock assessment and am having problems with this line of code 

```
def BullsAndCows():
Guess= input("Please enter a 4 digit number, remember no duplicates!")
Guess = str(Guess)
while len(Guess) != 4:
    Guess = input("IT has to be FOUR digits!")
    Guess = str(Guess)

while Guess[0] == Guess[1] or Guess[0] == Guess[2] or Guess[0] == Guess[3] or Guess[1] == Guess[2] or Guess[1] == Guess[3] or Guess[2] == Guess[3]:
    Guess = input("You can't use duplicates silly! Try another number!")
    Guess = str(Guess)`

```

The problem is if i input a 4 digit number with a duplicate i can no longer input a non 4 digit number without it outputting this error

> 
> Traceback (most recent call last):
>  File "python", line 64, in 
>  File "python", line 57, in BullsAndCows
>  IndexError: string index out of range
> 
> 
> 

Line 57 is the `while Guess[0] ==`...

Line 64 is just `BullsandCows()` which is used to call the function.
Anyone know the problem?