diff --git a/NumLib/DOF/LocalToGlobalIndexMap.h b/NumLib/DOF/LocalToGlobalIndexMap.h
index 7794a39e845d1a024d20a4636d354371f004af52..2ec3b7abf04f43f777552a6a68be455a77cb810f 100644
--- a/NumLib/DOF/LocalToGlobalIndexMap.h
+++ b/NumLib/DOF/LocalToGlobalIndexMap.h
@@ -44,7 +44,7 @@ class LocalToGlobalIndexMap final
 {
     // Enables using std::make_unique with private constructors from within
     // member functions of LocalToGlobalIndexMap. Cf.
-    // https://seanmiddleditch.com/enabling-make-unique-with-private-constructors/
+    // https://web.archive.org/web/20210302195245/https://seanmiddleditch.com/enabling-make-unique-with-private-constructors/
     struct ConstructorTag
     {
         explicit ConstructorTag() = default;