Why is this recursive function not returning anything?

So I have made a function to traverse through a linked list, recursively and at every traversal I concatenate the value to a string (empty string during initialization), the function seems to be working properly. I added a print statement in the base case to see what the value of the variable before the return statement, here is the code

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
def addTwoNumbers(self, l1, l2):
    s1 = ""
    s2 = ""
    def num(l, s):
        if(l==None):
            print(s)
            return s
        s = s+(str(l.val))
        num(l.next, s)
    print(num(l1,s1))

This is the Output