diff --git a/test/ShutdownHandler.hxx b/test/ShutdownHandler.hxx
index e4e900e5d..9db88a1b4 100644
--- a/test/ShutdownHandler.hxx
+++ b/test/ShutdownHandler.hxx
@@ -24,7 +24,7 @@ class EventLoop;
 
 class ShutdownHandler {
 public:
-	ShutdownHandler(EventLoop &loop);
+	explicit ShutdownHandler(EventLoop &loop);
 	~ShutdownHandler();
 };