diff --git a/src/include/lwip/sys.h b/src/include/lwip/sys.h
index 38d93ad1..052ee046 100644
--- a/src/include/lwip/sys.h
+++ b/src/include/lwip/sys.h
@@ -44,6 +44,7 @@ extern "C" {
    definitions of the sys_ functions. */
 typedef u8_t sys_sem_t;
 typedef u8_t sys_mbox_t;
+typedef u8_t sys_prot_t;
 struct sys_timeo {u8_t dummy;};
 
 #define sys_init()