ما هو حل الخطأ IndexError: list index out of range في بايثون؟

def decrypt(self,block):
    
    roundkeys = []
    state = string2number(block)
    
    for i in range(1, 31):
       state = addRoundKey(state, roundkeys[-i-1])
       state = pLayer_dec(state)
       state = sBoxLayer_dec(state)
    
    decipher = addRoundKey(state, roundkeys[0])
    
    return number2string_N(decipher,8)

state = addRoundKey(state, roundkeys[-i-1])
IndexError: list index out of range