diff --git a/video/out/x11_common.c b/video/out/x11_common.c
index 01611e0326..40d6b77420 100644
--- a/video/out/x11_common.c
+++ b/video/out/x11_common.c
@@ -2250,7 +2250,7 @@ bool vo_x11_screen_is_composited(struct vo *vo)
 {
     struct vo_x11_state *x11 = vo->x11;
     char buf[50];
-    sprintf(buf, sizeof(buf), "_NET_WM_CM_S%d", x11->screen);
+    snprintf(buf, sizeof(buf), "_NET_WM_CM_S%d", x11->screen);
     Atom NET_WM_CM = XInternAtom(x11->display, buf, False);
     return XGetSelectionOwner(x11->display, NET_WM_CM) != None;
 }