Leetcode problems solution throwing error please assist MY CODE RUNS JUST FINE I DONT GET WHERE THE ISSUE IS COMMING FROM PLS ASSIST ME SIR

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself. example

Example one
Input: l1 = [2,4,3], l2 = [5,6,4]
Output: [7,0,8]
Explanation: 342 + 465 = 807.
Example 2:

Input: l1 = [0], l2 = [0]
Output: [0]



MY CODE

var addTwoNumbers = function(l1, l2) {

   var alpha1 = "" 
   var alpha2 = ""
   var arr = []


 // Looping through first list l1

   for(var i = 0; i< l1.length; i++){
    alpha1 = l1[i]+alpha1

    // converting   l1 to string and setting it to ALPHA 1 
   }

   // Looping through second  list L2
   for(var i = 0; i< l2.length; i++){
    alpha2 = l2[i]+alpha2

    // converting   l2 to string and setting it to ALPHA 2 
   }
   // Coverting alpha1 and apha2 to intergets and making their sum to a new variable string "New"
  var New = parseInt(alpha1) +  parseInt(alpha2)
   New = New + ""
 
  for(var i=0;i<New.length; i++){
// Pushing the New variable is reverse order to arr and also converting it back to interger
arr.push(parseInt(New[New.length-i-1]))

  }
  // Returning the array
  return arr
};
addTwoNumbers([9,9,9,9,9,9,9],[9,9,9,9])

MY CODE WORKS JUST FINE ON CONSOLE

THE ERROR

Line 51 in solution.js
             throw new TypeError(__serialize__(ret) + " is not valid value for the expected return type ListNode");
             ^
TypeError: [null,null,null] is not valid value for the expected return type ListNode
    Line 51: Char 20 in solution.js (Object.<anonymous>)
    Line 16: Char 8 in runner.js (Object.runner)
    Line 35: Char 26 in solution.js (Object.<anonymous>)
    Line 1251: Char 30 in loader.js (Module._compile)
    Line 1272: Char 10 in loader.js (Object.Module._extensions..js)
    Line 1100: Char 32 in loader.js (Module.load)
    Line 962: Char 14 in loader.js (Function.Module._load)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:72:12)
    Line 17: Char 47 in run_main_module.js