diff --git a/slirp/mbuf.c b/slirp/mbuf.c
index 0c189e1a7bf1b931a39f5321d4d1788a664eda94..1b7868355a38554ca9b47726d73a08aefcd13538 100644
--- a/slirp/mbuf.c
+++ b/slirp/mbuf.c
@@ -154,7 +154,7 @@ m_inc(struct mbuf *m, int size)
     int datasize;
 
     /* some compilers throw up on gotos.  This one we can fake. */
-    if (m->m_size > size) {
+    if (M_ROOM(m) > size) {
         return;
     }