--- kernel/bsd/miscfs/union/union_subr.c	2018/04/24 18:26:01	1.1
+++ kernel/bsd/miscfs/union/union_subr.c	2018/04/24 18:27:58	1.1.1.2
@@ -956,7 +956,8 @@ union_vn_create(vpp, un, p)
 		return (error);
 	}
 
-	vp->v_writecount++;
+	if (++vp->v_writecount <= 0)
+		panic("union: v_writecount");
 	*vpp = vp;
 	return (0);
 }