making a RC4 algorithm, problems with the KSA(key scheduling algorithm)

Question

this is my code:

def KSA(key):
    key_length = len(key)
    S = list(range(256))
    j = 0
    for i in range(0,255):
        j = (j + S[i] + key[i % key_length]) & 256
        print(S[i],S[j])
        S[i] , S[j] = S[j] , S[i]  # Swapping
    return S

getting list index out of range when used with key = [5]. and i don't know why.