RuntimeError: dictionary changed size during iteration about mugo HOT 3 OPEN

def score(self):
    'Return score from B perspective. If W is winning, score is negative.'
    working_board = np.copy(self.board)
    while EMPTY in working_board:
        unassigned_spaces = np.where(working_board == EMPTY)
        c = unassigned_spaces[0][0], unassigned_spaces[1][0]
        territory, borders = find_reached(working_board, c)
        border_colors = set(working_board[b] for b in borders)
        X_border = BLACK in border_colors
        O_border = WHITE in border_colors
        if X_border and not O_border:
            territory_color = BLACK
        elif O_border and not X_border:
            territory_color = WHITE
        else:
            territory_color = UNKNOWN  # dame, or seki
        place_stones(working_board, territory_color, territory)

    return np.count_nonzero(working_board == BLACK) - np.count_nonzero(working_board == WHITE) - self.komi