diff --git a/sys/sys/mbuf.h b/sys/sys/mbuf.h
index 2d49f1b2c42c..d59ab541796e 100644
--- a/sys/sys/mbuf.h
+++ b/sys/sys/mbuf.h
@@ -283,7 +283,10 @@ struct mcntfree_lst {
  */
 #define MEXT_IS_REF(m) ((m)->m_ext.ref_cnt->refcnt > 1)
 
-#define MEXT_REM_REF(m) atomic_subtract_long(&((m)->m_ext.ref_cnt->refcnt), 1)
+#define MEXT_REM_REF(m) do {						\
+	KASSERT((m)->m_ext.ref_cnt->refcnt > 0, ("m_ext refcnt < 0"));	\
+	atomic_subtract_long(&((m)->m_ext.ref_cnt->refcnt), 1);		\
+	} while(0)
 
 #define MEXT_ADD_REF(m) atomic_add_long(&((m)->m_ext.ref_cnt->refcnt), 1)