diff --git a/src/api/api_msg.c b/src/api/api_msg.c
index 73ccbfef..fdb1a207 100644
--- a/src/api/api_msg.c
+++ b/src/api/api_msg.c
@@ -528,6 +528,13 @@ netconn_alloc(enum netconn_type t, netconn_callback callback)
   conn->socket       = -1;
   conn->callback     = callback;
   conn->recv_avail   = 0;
+#if LWIP_TCP
+  conn->write_msg    = NULL;
+  conn->write_offset = 0;
+# if LWIP_TCPIP_CORE_LOCKING
+  conn->write_delayed = 0;
+# endif /* LWIP_TCPIP_CORE_LOCKING */
+#endif /* LWIP_TCP */
 #if LWIP_SO_RCVTIMEO
   conn->recv_timeout = 0;
 #endif /* LWIP_SO_RCVTIMEO */
diff --git a/src/include/lwip/api.h b/src/include/lwip/api.h
index 323ac1b6..ecd22a19 100644
--- a/src/include/lwip/api.h
+++ b/src/include/lwip/api.h
@@ -138,20 +138,20 @@ struct netconn {
   int recv_bufsize;
 #endif /* LWIP_SO_RCVBUF */
   u16_t recv_avail;
+#if LWIP_TCP
   /** TCP: when data passed to netconn_write doesn't fit into the send buffer,
       this temporarily stores the message. */
   struct api_msg_msg *write_msg;
-#if LWIP_TCP
   /** TCP: when data passed to netconn_write doesn't fit into the send buffer,
       this temporarily stores how much is already sent. */
   size_t write_offset;
-#endif /* LWIP_TCP */
-#if LWIP_TCPIP_CORE_LOCKING
+# if LWIP_TCPIP_CORE_LOCKING
   /** TCP: when data passed to netconn_write doesn't fit into the send buffer,
       this temporarily stores whether to wake up the original application task
       if data couldn't be sent in the first try. */
   u8_t write_delayed;
-#endif /* LWIP_TCPIP_CORE_LOCKING */
+# endif /* LWIP_TCPIP_CORE_LOCKING */
+#endif /* LWIP_TCP */
   /** A callback function that is informed about events for this netconn */
   netconn_callback callback;
 };