diff --git a/tiramisu/option/netmaskoption.py b/tiramisu/option/netmaskoption.py
index 0e4d196..ce6e74c 100644
--- a/tiramisu/option/netmaskoption.py
+++ b/tiramisu/option/netmaskoption.py
@@ -54,21 +54,21 @@ class NetmaskOption(StrOption):
                               vals,
                               warnings_only,
                               context):
-        #opts must be (netmask, network) options
         if context is undefined and len(vals) != 2:
             raise ConfigError(_('network_netmask needs a network and a netmask'))
         if None in vals or len(vals) != 2:
             return
         val_netmask, val_network = vals
+        opt_netmask, opt_network = opts
         try:
             ip_network('{0}/{1}'.format(val_network, val_netmask))
         except ValueError:
-            if current_opt == opts[1]:
+            if current_opt == opt_network:
                 raise ValueError(_('with netmask "{0}" ("{1}")').format(val_netmask,
-                                                                        opts[0].impl_get_display_name()))
+                                                                        opt_netmask.impl_get_display_name()))
             else:
                 raise ValueError(_('with network "{0}" ("{1}")').format(val_network,
-                                                                        opts[1].impl_get_display_name()))
+                                                                        opt_network.impl_get_display_name()))
 
     def _cons_ip_netmask(self,
                          current_opt,