diff --git a/video/out/vo_opengl_old.c b/video/out/vo_opengl_old.c
index 8c32676e1d..5943df10b4 100644
--- a/video/out/vo_opengl_old.c
+++ b/video/out/vo_opengl_old.c
@@ -26,6 +26,7 @@
 #include <string.h>
 #include <math.h>
 #include <stdbool.h>
+#include <ctype.h>
 #include <assert.h>
 
 #include "config.h"