diff --git a/include/boost/smart_ptr/intrusive_ptr.hpp b/include/boost/smart_ptr/intrusive_ptr.hpp
index e5db609..58041da 100644
--- a/include/boost/smart_ptr/intrusive_ptr.hpp
+++ b/include/boost/smart_ptr/intrusive_ptr.hpp
@@ -59,7 +59,7 @@ public:
 
     typedef T element_type;
 
-    intrusive_ptr() BOOST_NOEXCEPT : px( 0 )
+    BOOST_CONSTEXPR intrusive_ptr() BOOST_NOEXCEPT : px( 0 )
     {
     }