diff --git a/src/netif/ppp/fsm.c b/src/netif/ppp/fsm.c
index ee0c708c..3e63e2fd 100644
--- a/src/netif/ppp/fsm.c
+++ b/src/netif/ppp/fsm.c
@@ -751,6 +751,7 @@ static void fsm_sconfreq(fsm *f, int retransmit) {
     PUTSHORT(cilen + HEADERLEN, outp);
     if (cilen != 0) {
 	(*f->callbacks->addci)(f, outp, &cilen);
+	LWIP_ASSERT("cilen == p->len - HEADERLEN - PPP_HDRLEN", cilen == p->len - HEADERLEN - PPP_HDRLEN);
     }
 
     ppp_write(pcb, p);