Linked List method has non-stopping runtime

Question

I implemented a linked list method that inserts another Linked List after a particular index and then continues with other elements that were originally after this index.

def insert_list(self, other: LinkedList, i: int) -> None:
        """
        >>> linky = LinkedList([0, 1, 2, 3, 4])
        >>> other = LinkedList([100, 101])
        >>> linky.insert_list(other, 3)
        >>> print(linky)
        [0 -> 1 -> 2 -> 3 -> 100 -> 101 -> 4]
        >>> print(other)
        [100 -> 101]
        """
        curr1 = self._first
        curr2 = other._first
        j = 0
        while curr1 is not None:
            if i == j:
                after = curr1.next
                while curr2 is not None:
                    curr1.next = curr2
                    curr1 = curr1.next
                    curr2 = curr2.next
                curr1.next = after
            else:
                j += 1
                curr1 = curr1.next
        return None

But when I run my doctests they are running infinitely, I guess there is a mistake in my code. Can anyone point out what is wrong?