diff --git a/include/MacStd/common.hpp b/include/MacStd/common.hpp
index 00d340a..497e804 100644
--- a/include/MacStd/common.hpp
+++ b/include/MacStd/common.hpp
@@ -323,6 +323,10 @@ namespace macstd
 			}
 		}
 
+	private:
+		// Force use of valid() method when testing handle.
+		operator bool() const;
+
 	protected:
 		T m_h; ///< Object handle
 	};