diff --git a/src/include/lwip/sys.h b/src/include/lwip/sys.h
index 794f38f5..8c05cf42 100644
--- a/src/include/lwip/sys.h
+++ b/src/include/lwip/sys.h
@@ -47,6 +47,7 @@ struct sys_timeout {u8_t dummy;};
 
 #define sys_init()
 #define sys_timeout(m,h,a)
+#define sys_untimeout(m,a)
 #define sys_sem_new(c) c
 #define sys_sem_signal(s)
 #define sys_sem_wait(s)