合并K排序列表中的大小为2的列表出错

import heapq

def merge_lists(lists):
    # 检查输入的K个排序列表是否正确
    for lst in lists:
        if not lst:
            return []
        if lst != sorted(lst):
            lst.sort()

    result = []
    min_heap = []

    # 初始化最小堆
    for i, lst in enumerate(lists):
        heapq.heappush(min_heap, (lst[0], i, 0))

    # 从最小堆中选择最小的元素，并将其添加到结果列表中
    while min_heap:
        val, list_index, element_index = heapq.heappop(min_heap)
        result.append(val)

        # 将下一个元素插入到最小堆中
        if element_index + 1 < len(lists[list_index]):
            heapq.heappush(min_heap, (lists[list_index][element_index + 1], list_index, element_index + 1))

    return result

# 示例输入
lists = [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
# 调用合并函数
merged_list = merge_lists(lists)
print(merged_list)