diff --git a/include/memory b/include/memory
index 41ab01b..bb1b415 100644
--- a/include/memory
+++ b/include/memory
@@ -3924,7 +3924,10 @@
 
     template <class _Yp, class _OrigPtr>
         _LIBCPP_INLINE_VISIBILITY
-        void
+        typename enable_if<is_convertible<_OrigPtr*,
+                                          const enable_shared_from_this<_Yp>*
+        >::value,
+            void>::type
         __enable_weak_this(const enable_shared_from_this<_Yp>* __e,
                            _OrigPtr* __ptr) _NOEXCEPT
         {
@@ -3943,6 +3946,7 @@
     template <class _Up> friend class _LIBCPP_TEMPLATE_VIS weak_ptr;
 };
 
+
 template<class _Tp>
 inline
 _LIBCPP_CONSTEXPR