diff --git a/src/api/api_msg.c b/src/api/api_msg.c
index 4830a468..81bc93d5 100644
--- a/src/api/api_msg.c
+++ b/src/api/api_msg.c
@@ -221,6 +221,7 @@ recv_udp(void *arg, struct udp_pcb *pcb, struct pbuf *p,
   struct netbuf *buf;
   struct netconn *conn;
   u16_t len;
+  err_t err;
 #if LWIP_SO_RCVBUF
   int recv_avail;
 #endif /* LWIP_SO_RCVBUF */
@@ -269,8 +270,10 @@ recv_udp(void *arg, struct udp_pcb *pcb, struct pbuf *p,
   }
 
   len = p->tot_len;
-  if (sys_mbox_trypost(&conn->recvmbox, buf) != ERR_OK) {
+  err = sys_mbox_trypost(&conn->recvmbox, buf);
+  if (err != ERR_OK) {
     netbuf_delete(buf);
+    LWIP_DEBUGF(API_MSG_DEBUG, ("recv_udp: sys_mbox_trypost failed, err=%d\n", err));
     return;
   } else {
 #if LWIP_SO_RCVBUF