Index: sys/netpfil/pf/pf.c
===================================================================
--- sys/netpfil/pf/pf.c
+++ sys/netpfil/pf/pf.c
@@ -5571,6 +5571,9 @@
 	/* Copied from FreeBSD 10.0-CURRENT ip_output. */
 	m0->m_pkthdr.csum_flags |= CSUM_IP;
 	if (m0->m_pkthdr.csum_flags & CSUM_DELAY_DATA & ~ifp->if_hwassist) {
+		m0 = mb_unmapped_to_ext(m0);
+		if (m0 == NULL)
+			goto done;
 		in_delayed_cksum(m0);
 		m0->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA;
 	}