--- a/netwerk/sctp/src/user_mbuf.c
+++ b/netwerk/sctp/src/user_mbuf.c
@@ -982,16 +982,23 @@ m_copym(struct mbuf *m, int off0, int le
 {
 	struct mbuf *n, **np;
 	int off = off0;
 	struct mbuf *top;
 	int copyhdr = 0;
 
 	KASSERT(off >= 0, ("m_copym, negative off %d", off));
 	KASSERT(len >= 0, ("m_copym, negative len %d", len));
+	KASSERT(m != NULL, ("m_copym, m is NULL"));
+
+#if !defined(INVARIANTS)
+	if (m == NULL) {
+		return (NULL);
+	}
+#endif
 
 	if (off == 0 && m->m_flags & M_PKTHDR)
 		copyhdr = 1;
 	while (off > 0) {
 		KASSERT(m != NULL, ("m_copym, offset > size of mbuf chain"));
 		if (off < m->m_len)
 			break;
 		off -= m->m_len;