diff --git a/slirp/mbuf.c b/slirp/mbuf.c
index eadc802..ce2eb84 100644
--- a/slirp/mbuf.c
+++ b/slirp/mbuf.c
@@ -65,7 +65,7 @@
 	m->m_flags = (flags | M_USEDLIST);
 
 	/* Initialise it */
-	m->m_size = SLIRP_MSIZE - offsetof(struct m_hdr, m_dat);
+	m->m_size = SLIRP_MSIZE - offsetof(struct mbuf, m_dat);
 	m->m_data = m->m_dat;
 	m->m_len = 0;
         m->m_nextpkt = NULL;