diff --git a/src/api/api_lib.c b/src/api/api_lib.c
index abf4edb0..3a87c297 100644
--- a/src/api/api_lib.c
+++ b/src/api/api_lib.c
@@ -85,8 +85,10 @@ netconn_apimsg(tcpip_callback_fn fn, struct api_msg *apimsg)
   apimsg->op_completed_sem = LWIP_NETCONN_THREAD_SEM_GET();
 #endif /* LWIP_NETCONN_SEM_PER_THREAD */
   
-  tcpip_send_msg_wait_sem(fn, apimsg, LWIP_API_MSG_SEM(apimsg));
-  return apimsg->err;
+  if (tcpip_send_msg_wait_sem(fn, apimsg, LWIP_API_MSG_SEM(apimsg)) == ERR_OK) {
+    return apimsg->err;
+  }
+  return ERR_VAL;
 }
 
 /**
@@ -396,7 +398,7 @@ netconn_accept(struct netconn *conn, struct netconn **new_conn)
   /* Let the stack know that we have accepted the connection. */
   API_MSG_VAR_ALLOC_DONTFAIL(msg);
   API_MSG_VAR_REF(msg).conn = newconn;
-  /* don't care for the return value of lwip_netconn_do_accepted */
+  /* don't care for the return value of lwip_netconn_do_recv */
   netconn_apimsg(lwip_netconn_do_accepted, &API_MSG_VAR_REF(msg));
   API_MSG_VAR_FREE(msg);
 #endif /* TCP_LISTEN_BACKLOG */
diff --git a/src/api/tcpip.c b/src/api/tcpip.c
index a4507918..14ab29c0 100644
--- a/src/api/tcpip.c
+++ b/src/api/tcpip.c
@@ -314,8 +314,9 @@ tcpip_untimeout(sys_timeout_handler h, void *arg)
  * @param fn function to be called from TCPIP thread
  * @param apimsg argument to API function
  * @param sem semaphore to wait on
+ * @return ERR_OK if the function was called, another err_t if not
  */
-void
+err_t
 tcpip_send_msg_wait_sem(tcpip_callback_fn fn, void *apimsg, sys_sem_t* sem)
 {
 #if LWIP_TCPIP_CORE_LOCKING
@@ -323,6 +324,7 @@ tcpip_send_msg_wait_sem(tcpip_callback_fn fn, void *apimsg, sys_sem_t* sem)
   LOCK_TCPIP_CORE();
   fn(apimsg);
   UNLOCK_TCPIP_CORE();
+  return ERR_OK;
 #else /* LWIP_TCPIP_CORE_LOCKING */
   LWIP_ASSERT("semaphore not initialized", sys_sem_valid(sem));
 
@@ -337,6 +339,7 @@ tcpip_send_msg_wait_sem(tcpip_callback_fn fn, void *apimsg, sys_sem_t* sem)
   sys_mbox_post(&mbox, &TCPIP_MSG_VAR_REF(msg));
   sys_arch_sem_wait(sem, 0);
   TCPIP_MSG_VAR_FREE(msg);
+  return ERR_OK;
 #endif /* LWIP_TCPIP_CORE_LOCKING */
 }
 
diff --git a/src/include/lwip/priv/tcpip_priv.h b/src/include/lwip/priv/tcpip_priv.h
index 278c1103..c754e3d0 100644
--- a/src/include/lwip/priv/tcpip_priv.h
+++ b/src/include/lwip/priv/tcpip_priv.h
@@ -92,7 +92,7 @@ struct netif;
 #define API_MSG_M_DEF_C(t, m)           const t * m
 #endif /* LWIP_MPU_COMPATIBLE */
 
-void tcpip_send_msg_wait_sem(tcpip_callback_fn fn, void *apimsg, sys_sem_t* sem);
+err_t tcpip_send_msg_wait_sem(tcpip_callback_fn fn, void *apimsg, sys_sem_t* sem);
 
 struct tcpip_api_call_data
 {