diff --git a/sys/kern/uipc_mbuf.c b/sys/kern/uipc_mbuf.c
index a16af57a85b3..01e4c310cb19 100644
--- a/sys/kern/uipc_mbuf.c
+++ b/sys/kern/uipc_mbuf.c
@@ -934,6 +934,7 @@ m_dup(struct mbuf *m, int how)
 			}
 			if ((n->m_flags & M_EXT) == 0)
 				nsize = MHLEN;
+			n->m_flags &= ~M_RDONLY;
 		}
 		n->m_len = 0;