diff --git a/lwip/src/arch/sys_arch.c b/lwip/src/arch/sys_arch.c
index 4df052db..594cf38d 100644
--- a/lwip/src/arch/sys_arch.c
+++ b/lwip/src/arch/sys_arch.c
@@ -191,7 +191,11 @@ void sys_mbox_free(sys_mbox_t* mbox)
  */
 u32_t sys_arch_mbox_tryfetch(sys_mbox_t* mbox, void** msg)
 {
-	return mailbox_ptr_tryfetch(&mbox->mailbox, msg);
+	int ret = mailbox_ptr_tryfetch(&mbox->mailbox, msg);
+
+	if (ret)
+		return SYS_MBOX_EMPTY;
+	return 0;
 }
 
 /* sys_mbox_new(): create a new mailbox with a minimum size of "size"
@@ -223,9 +227,10 @@ err_t sys_mbox_trypost(sys_mbox_t *mbox, void *msg)
 	err = mailbox_ptr_trypost(&mbox->mailbox, msg);
 	if (err != 0) {
 		LWIP_DEBUGF(SYS_DEBUG, ("sys_mbox_trypost: %d\n", err));
+		return ERR_MEM;
 	}
 
-	return err;
+	return ERR_OK;
 }
 
 /* sys_mbox_post(): post new data to the mailbox
@@ -259,7 +264,7 @@ void sys_mutex_unlock(sys_mutex_t* mutex)
 err_t sys_mutex_new(sys_mutex_t * mutex)
 {
 	sem_init(mutex, 1);
-	return 0;
+	return ERR_OK;
 }
 
 #if SYS_LIGHTWEIGHT_PROT
@@ -267,7 +272,7 @@ err_t sys_mutex_new(sys_mutex_t * mutex)
 sys_prot_t sys_arch_protect(void)
 {
 	spinlock_irqsave_lock(&lwprot_lock);
-	return 0;
+	return ERR_OK;
 }
 
 void sys_arch_unprotect(sys_prot_t pval)