diff --git a/libs/gtkmm2ext/gtkmm2ext/dndvbox.h b/libs/gtkmm2ext/gtkmm2ext/dndvbox.h
index 92ec8f960f..b4c84567d7 100644
--- a/libs/gtkmm2ext/gtkmm2ext/dndvbox.h
+++ b/libs/gtkmm2ext/gtkmm2ext/dndvbox.h
@@ -19,6 +19,8 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#pragma once
+
 #include <gtkmm/window.h>
 #include <gtkmm/box.h>