diff --git a/sys/netgraph/ng_base.c b/sys/netgraph/ng_base.c
index ec1b6a5ff5f1..200552b944b8 100644
--- a/sys/netgraph/ng_base.c
+++ b/sys/netgraph/ng_base.c
@@ -330,8 +330,12 @@ static	ng_ID_t nextID = 1;
 		int total;						\
 									\
 		M_ASSERTPKTHDR(m);					\
-		for (total = 0, n = (m); n != NULL; n = n->m_next)	\
+		for (total = 0, n = (m); n != NULL; n = n->m_next) {	\
 			total += n->m_len;				\
+			if (n->m_nextpkt != NULL)			\
+				panic("%s: m_nextpkt", __func__);	\
+		}							\
+
 		if ((m)->m_pkthdr.len != total) {			\
 			panic("%s: %d != %d",				\
 			    __func__, (m)->m_pkthdr.len, total);	\