diff --git a/sys/kern/uipc_mbuf.c b/sys/kern/uipc_mbuf.c
--- a/sys/kern/uipc_mbuf.c
+++ b/sys/kern/uipc_mbuf.c
@@ -1280,6 +1280,8 @@
 
 	if (m0 == NULL)
 		return;
+
+	MPASS(M_WRITABLE(m0));
 	while (off > (mlen = m->m_len)) {
 		off -= mlen;
 		totlen += mlen;