python中象棋引擎的Minimax函数无法解包不可iterable的非类型对象

我正在尝试使用chess包用python构建一个简单的国际象棋引擎。我的minimax函数有问题,会破坏atm。下面是我的代码。。。

# import package
import chess

# define starting params
board = chess.Board()
depth = 3
alpha = -1000000
beta = 1000000
player = True

# board evaluation
def boardEval(board):
    wp = len(board.pieces(chess.PAWN, chess.WHITE))
    bp = len(board.pieces(chess.PAWN, chess.BLACK))
    wn = len(board.pieces(chess.KNIGHT, chess.WHITE))
    bn = len(board.pieces(chess.KNIGHT, chess.BLACK))
    wb = len(board.pieces(chess.BISHOP, chess.WHITE))
    bb = len(board.pieces(chess.BISHOP, chess.BLACK))
    wr = len(board.pieces(chess.ROOK, chess.WHITE))
    br = len(board.pieces(chess.ROOK, chess.BLACK))
    wq = len(board.pieces(chess.QUEEN, chess.WHITE))
    bq = len(board.pieces(chess.QUEEN, chess.BLACK))
    return (wp-bp) + 3 * (wn-bn) + 3 * (wb-bb) - 5 * (wr-br) + 10 * (wq-bq)


# minimax function
def minimax(board, depth, alpha, beta, player):
    # check whether game is complete
    gameComplete = board.is_game_over()
    bestMove = None
    moves = [str(l) for l in board.legal_moves]
    tempBoard = board.copy()
    
    if depth == 0:
        return boardEval(tempBoard), bestMove
    if gameComplete:
        print('Game completed')
        return None

    for move in  moves:
        m = chess.Move.from_uci(move)
        tempBoard.push(m)
        
        if player:
            maxEval = -1000000
            value, bestMove = minimax(tempBoard, depth - 1, alpha, beta, False)
            if maxEval < value:
                maxEval = value
                bestMove = move
            alpha = alpha if alpha > value else value
            if beta <= alpha:
                break
            return maxEval, bestMove
        else:
            minEval = 1000000
            value, bestMove = minimax(tempBoard, depth - 1, alpha, beta, True)
            if minEval < value:
                minEval = value
                bestMove = move
            beta = beta if beta < value else value
            if beta <= alpha:
                break
            return minEval, bestMove
       
# find best move
minimax(board, depth=3, alpha=-100000, beta=100000, player=True)

但是,在运行minimax函数时,我收到以下错误消息:

Traceback (most recent call last):
  File "<ipython-input-10-9ddb4749c86c>", line 64, in <module>
    minimax(board, depth=3, alpha=-100000, beta=100000, player=True)
  File "<ipython-input-10-9ddb4749c86c>", line 44, in minimax
    value, bestMove = minimax(tempBoard, depth - 1, alpha, beta, False)
TypeError: cannot unpack non-iterable NoneType object

我不明白为什么会发生这种情况,也不知道如何解决。有什么想法吗?