diff --git a/sys/netgraph/ng_tty.c b/sys/netgraph/ng_tty.c
--- a/sys/netgraph/ng_tty.c
+++ b/sys/netgraph/ng_tty.c
@@ -485,9 +485,7 @@
 	}
 
 	/* Add char to mbuf */
-	*mtod(m, u_char *) = c;
-	m->m_data++;
-	m->m_len++;
+	*(u_char *)mtodo(m, m->m_len++) = c;
 	m->m_pkthdr.len++;
 
 	/* Ship off mbuf if it's time */