diff --git a/include/boost/fiber/fiber.hpp b/include/boost/fiber/fiber.hpp
index bb8a39ca..295763b7 100644
--- a/include/boost/fiber/fiber.hpp
+++ b/include/boost/fiber/fiber.hpp
@@ -71,7 +71,7 @@ public:
         impl_()
     {}
 
-    fiber( ptr_t imp) BOOST_NOEXCEPT :
+    explicit fiber( ptr_t imp) BOOST_NOEXCEPT :
         impl_( imp)
     {}