diff --git a/MoreLinq/TagFirstLast.cs b/MoreLinq/TagFirstLast.cs
index f22d3fbbf..2ffb5c9a3 100644
--- a/MoreLinq/TagFirstLast.cs
+++ b/MoreLinq/TagFirstLast.cs
@@ -60,8 +60,24 @@ public static IEnumerable<TResult> TagFirstLast<TSource, TResult>(this IEnumerab
             if (source == null) throw new ArgumentNullException(nameof(source));
             if (resultSelector == null) throw new ArgumentNullException(nameof(resultSelector));
 
-            return source.Index() // count-up
-                         .CountDown(1, (e, cd) => resultSelector(e.Value, e.Key == 0, cd == 0));
+            return _(); IEnumerable<TResult> _()
+            {
+                using var enumerator = source.GetEnumerator();
+
+                if (!enumerator.MoveNext())
+                    yield break;
+
+                var current = enumerator.Current;
+                var hasNext = enumerator.MoveNext();
+                yield return resultSelector(current, true, !hasNext);
+
+                while (hasNext)
+                {
+                    current = enumerator.Current;
+                    hasNext = enumerator.MoveNext();
+                    yield return resultSelector(current, false, !hasNext);
+                }
+            }
         }
     }
 }