diff --git a/sys/net/if_tun.c b/sys/net/if_tun.c
index 065f6c9b055a..9b07c3c4a0a4 100644
--- a/sys/net/if_tun.c
+++ b/sys/net/if_tun.c
@@ -765,7 +765,7 @@ tunwrite(struct cdev *dev, struct uio *uio, int flag)
 		return (ENOBUFS);
 	mlen = MHLEN;
 
-	top = 0;
+	top = NULL;
 	mp = ⊤
 	while (error == 0 && uio->uio_resid > 0) {
 		m->m_len = min(mlen, uio->uio_resid);