diff --git a/include/boost/weak_ptr.hpp b/include/boost/weak_ptr.hpp
index 87231c3..d73c5f3 100644
--- a/include/boost/weak_ptr.hpp
+++ b/include/boost/weak_ptr.hpp
@@ -49,6 +49,11 @@ public:
 
 //  generated copy constructor, assignment, destructor are fine
 
+    template<typename Y>
+    weak_ptr(weak_ptr<Y> const & r): px(r.px), pn(r.pn) // never throws
+    {
+    }
+
     template<typename Y>
     weak_ptr(shared_ptr<Y> const & r): px(r.px), pn(r.pn) // never throws
     {