“'str'对象不支持项目分配”

我不明白这段代码有什么问题。我运行它的时间大约有一半，它给了我一个'str'对象，不支持项目分配错误。字符串应不重叠。

from random import randint, sample

board = []


for x in range(0, 5):
    board.append([","] * 11)

board.append(",,,,,@,,,,,")

for x in range(0, 5):
    board.append([","] * 11)

def print_board(board):
    for row in board:
        print " ".join(row)

x = sample(range(1, 11), 6)
y = sample(range(1, 11), 6)




board[x[0]][y[0]] = "A"
board[x[1]][y[1]] = "B"
board[x[2]][y[2]] = "C"
board[x[3]][y[3]] = "D"
board[x[4]][y[4]] = "E"
board[x[5]][y[5]] = "F"

print_board(board)

任何帮助，将不胜感激，谢谢。