diff --git a/src/window_manager_private.h b/src/window_manager_private.h
index cb067a5..abbc4e8 100644
--- a/src/window_manager_private.h
+++ b/src/window_manager_private.h
@@ -1,6 +1,8 @@
 #ifndef WINDOW_MANAGER_PRIVATE_H
 #define WINDOW_MANAGER_PRIVATE_H
 
+#include <stddef.h>
+
 #include "geometry.h"
 
 namespace fallout {