diff --git a/window-border.c b/window-border.c
index adb81b8b5..5420f4b97 100644
--- a/window-border.c
+++ b/window-border.c
@@ -18,6 +18,7 @@
 
 #include <sys/types.h>
 
+#include <stdlib.h>
 #include <string.h>
 
 #include "tmux.h"