diff --git a/src/custom_types/enum/testcase_linked_list.md b/src/custom_types/enum/testcase_linked_list.md
index cd4e3e6202..8c41ed7c63 100644
--- a/src/custom_types/enum/testcase_linked_list.md
+++ b/src/custom_types/enum/testcase_linked_list.md
@@ -38,6 +38,7 @@ impl List {
         match *self {
             // Can't take ownership of the tail, because `self` is borrowed;
             // instead take a reference to the tail
+            // And it'a a non-tail recursive call which may cause stack overflow for long lists.
             Cons(_, ref tail) => 1 + tail.len(),
             // Base Case: An empty list has zero length
             Nil => 0