diff --git a/sys/net/if_vlan.c b/sys/net/if_vlan.c
index 5a8fafe6d421..758800acadf7 100644
--- a/sys/net/if_vlan.c
+++ b/sys/net/if_vlan.c
@@ -502,7 +502,7 @@ vlan_start(struct ifnet *ifp)
 				m_freem(m);
 				continue;
 			}
-			*(u_int*)(mtag + 1) = ifv->ifv_tag;
+			VLAN_TAG_VALUE(mtag) = ifv->ifv_tag;
 			m_tag_prepend(m, mtag);
 			m->m_flags |= M_VLANTAG;
 		} else {
diff --git a/sys/net/if_vlan_var.h b/sys/net/if_vlan_var.h
index 11cbf7b20fcf..c55d5d1ec555 100644
--- a/sys/net/if_vlan_var.h
+++ b/sys/net/if_vlan_var.h
@@ -96,6 +96,12 @@ struct	vlanreq {
 #define	MTAG_VLAN	1035328035
 #define	MTAG_VLAN_TAG	0		/* tag of VLAN interface */
 
+/*
+ * This macro must expand to a lvalue so that it can be used
+ * to set a tag with a simple assignment.
+ */
+#define	VLAN_TAG_VALUE(_mt)	(*(u_int *)((_mt) + 1))
+
 #define	VLAN_INPUT_TAG(_ifp, _m, _t, _errcase) do {		\
 	struct m_tag *mtag;					\
 	mtag = m_tag_alloc(MTAG_VLAN, MTAG_VLAN_TAG,		\
@@ -105,7 +111,7 @@ struct	vlanreq {
 		m_freem(_m);					\
 		_errcase;					\
 	}							\
-	*(u_int *)(mtag+1) = (_t);				\
+	VLAN_TAG_VALUE(mtag) = (_t);				\
 	m_tag_prepend((_m), mtag);				\
 	(_m)->m_flags |= M_VLANTAG;				\
 } while (0)
@@ -113,7 +119,6 @@ struct	vlanreq {
 #define	VLAN_OUTPUT_TAG(_ifp, _m)				\
 	((_m)->m_flags & M_VLANTAG ?				\
 		m_tag_locate((_m), MTAG_VLAN, MTAG_VLAN_TAG, NULL) : NULL)
-#define	VLAN_TAG_VALUE(_mt)	(*(u_int *)((_mt)+1))
 #endif /* _KERNEL */
 
 #endif /* _NET_IF_VLAN_VAR_H_ */