diff --git a/tiramisu/option.py b/tiramisu/option.py
index 313299d..0058866 100644
--- a/tiramisu/option.py
+++ b/tiramisu/option.py
@@ -718,7 +718,7 @@ class IPOption(Option):
     def _validate(self, value):
         ip = IP('{0}/32'.format(value))
         if ip.iptype() == 'RESERVED':
-            raise ValueError(_("IP mustn't not be in reserved class"))
+            raise ValueError(_("IP shall not be in reserved class"))
         if self._only_private and not ip.iptype() == 'PRIVATE':
             raise ValueError(_("IP must be in private class"))
 
@@ -800,7 +800,7 @@ class NetworkOption(Option):
     def _validate(self, value):
         ip = IP(value)
         if ip.iptype() == 'RESERVED':
-            raise ValueError(_("network mustn't not be in reserved class"))
+            raise ValueError(_("network shall not be in reserved class"))
 
 
 class NetmaskOption(Option):