diff --git a/src/Gui/DocumentObserver.h b/src/Gui/DocumentObserver.h
index ed2471b77b..4c550ac620 100644
--- a/src/Gui/DocumentObserver.h
+++ b/src/Gui/DocumentObserver.h
@@ -23,6 +23,7 @@
 #ifndef GUI_DOCUMENTOBSERVER_H
 #define GUI_DOCUMENTOBSERVER_H
 
+#include <Base/BaseClass.h>
 #include <boost_signals2.hpp>