diff --git a/applets/minimizeall/plugin/minimizeall.cpp b/applets/minimizeall/plugin/minimizeall.cpp
--- a/applets/minimizeall/plugin/minimizeall.cpp
+++ b/applets/minimizeall/plugin/minimizeall.cpp
@@ -26,6 +26,7 @@
 
 MinimizeAll::MinimizeAll()
 {
+    qRegisterMetaType<WId>("WId");
     m_active = false;
 }