Bug Report for merge-k-sorted-linked-lists

[NJiHin]

Bug Report for https://neetcode.io/problems/merge-k-sorted-linked-lists

Please describe the bug below and include any steps to reproduce the bug or screenshots if possible.
Missing teset case for this problem.
I tried submitted the below solution, which passed all test cases:

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

class Solution:    
    def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
        
        def merge2(l1, l2):
            dummy = ListNode()
            node = dummy

            while l1 and l2:
                if l1.val < l2.val:
                    node.next = l1
                    l1 = l1.next
                else:
                    node.next = l2
                    l2 = l2.next
                node = node.next
            if l1:
                node.next = l1
            elif l2:
                node.next = l2
            return dummy.next
        
        if len(lists) < 2:
            return None
        
        while len(lists) != 1:
            l1 = lists.pop()
            l2 = lists.pop()
            lists.append(merge2(l1,l2))
        
        return lists[0]

But running a test case such as [[5]] would cause it to fail.