diff --git a/neutron_tempest_plugin/api/base.py b/neutron_tempest_plugin/api/base.py
index 1b02211..77bf5e8 100644
--- a/neutron_tempest_plugin/api/base.py
+++ b/neutron_tempest_plugin/api/base.py
@@ -478,7 +478,7 @@
         """
 
         if not cls.try_reserve_subnet_cidr(addr, **ipnetwork_kwargs):
-            raise ValueError('Subnet CIDR already reserved: %r'.format(
+            raise ValueError('Subnet CIDR already reserved: {0!r}'.format(
                 addr))
 
     @classmethod