diff --git a/os/ospoll.c b/os/ospoll.c
index c68aabc87..387ead4e0 100644
--- a/os/ospoll.c
+++ b/os/ospoll.c
@@ -26,6 +26,7 @@
 
 #include <X11/X.h>
 #include <X11/Xproto.h>
+#include <assert.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include "misc.h"               /* for typedef of pointer */