diff --git a/2. Add Two Numbers/Solution.cs b/2. Add Two Numbers/Solution.cs
index 80cb7e5..dc3dbee 100644
--- a/2. Add Two Numbers/Solution.cs	
+++ b/2. Add Two Numbers/Solution.cs	
@@ -1,7 +1,7 @@
 namespace _2._Add_Two_Numbers;
 
 public class Solution {
-    public ListNode AddTwoNumbers(ListNode? l1, ListNode? l2)
+    public ListNode? AddTwoNumbers(ListNode? l1, ListNode? l2)
     {
         ListNode? answer = null;
         ListNode? answerEnd = answer;
@@ -24,7 +24,7 @@ public class Solution {
             toNext = sum / 10;
             sum %= 10;
             ListNode item = new ListNode(sum);
-            if (answer == null)
+            if (answer == null || answerEnd == null)
                 answer = answerEnd = item;
             else
             {
@@ -33,7 +33,7 @@ public class Solution {
             }
         }
 
-        if (toNext != 0)
+        if (toNext != 0 && answerEnd != null)
             answerEnd.next = new ListNode(toNext);
         return answer;
     }