diff --git a/lwip/sys_arch.c b/lwip/sys_arch.c
index 36a6eae..52f5330 100644
--- a/lwip/sys_arch.c
+++ b/lwip/sys_arch.c
@@ -284,6 +284,12 @@ err_t sys_mbox_trypost(sys_mbox_t *mbox, void *msg)
     return ERR_MEM;
 }
 
+err_t sys_mbox_trypost_fromisr(sys_mbox_t *q, void *msg)
+{
+    /* Quietly fail - not implemented. */
+    return ERR_MEM;
+}
+
 /*---------------------------------------------------------------------------*
  * Routine:  sys_arch_mbox_fetch
  *---------------------------------------------------------------------------*