diff --git a/include/boost/iostreams/detail/adapter/direct_adapter.hpp b/include/boost/iostreams/detail/adapter/direct_adapter.hpp
index b7300e4..19d182a 100755
--- a/include/boost/iostreams/detail/adapter/direct_adapter.hpp
+++ b/include/boost/iostreams/detail/adapter/direct_adapter.hpp
@@ -49,7 +49,7 @@ public:
           #endif
         { };
 protected:
-    direct_adapter_base(const Direct& d);
+    explicit direct_adapter_base(const Direct& d);
     typedef is_convertible<io_category, two_sequence> is_double;
     struct pointers {
         char_type *beg, *ptr, *end;