diff --git a/src/core/mem.c b/src/core/mem.c
index 004343d7..890a60c8 100644
--- a/src/core/mem.c
+++ b/src/core/mem.c
@@ -61,6 +61,7 @@
 #include "lwip/mem.h"
 #include "lwip/sys.h"
 #include "lwip/stats.h"
+#include "lwip/err.h"
 
 #include <string.h>
 
diff --git a/src/include/lwip/sys.h b/src/include/lwip/sys.h
index 0678b748..cec9578c 100644
--- a/src/include/lwip/sys.h
+++ b/src/include/lwip/sys.h
@@ -43,14 +43,19 @@ extern "C" {
 /* For a totally minimal and standalone system, we provide null
    definitions of the sys_ functions. */
 typedef u8_t sys_sem_t;
+typedef u8_t sys_mutex_t;
 typedef u8_t sys_mbox_t;
 
-#define sys_sem_new(c) ((sys_sem_t)c)
+#define sys_sem_new(s, c) ERR_OK
 #define sys_sem_signal(s)
 #define sys_sem_wait(s)
 #define sys_arch_sem_wait(s,t)
 #define sys_sem_free(s)
-#define sys_mbox_new(s) 0
+#define sys_mutex_new(mu) ERR_OK
+#define sys_mutex_lock(mu)
+#define sys_mutex_unlock(mu)
+#define sys_mutex_free(mu)
+#define sys_mbox_new(m, s) ERR_OK
 #define sys_mbox_fetch(m,d)
 #define sys_mbox_tryfetch(m,d)
 #define sys_mbox_post(m,d)
@@ -98,7 +103,7 @@ typedef void (*lwip_thread_fn)(void *arg);
 
 /** Create a new mutex
  * @param mutex pointer to the mutex to create
- * @return a new 
+ * @return a new mutex */
 err_t sys_mutex_new(sys_mutex_t *mutex);
 /** Lock a mutex
  * @param mutex the mutex to lock */