diff --git a/nova/exception.py b/nova/exception.py
index f96b1eaf3..b16bc7669 100644
--- a/nova/exception.py
+++ b/nova/exception.py
@@ -522,6 +522,10 @@ class PortNotFound(NotFound):
     message = _("Port %(port_id)s could not be found.")
 
 
+class PortNotUsable(NovaException):
+    message = _("Port %(port_id)s not usable for instance %(instance)s.")
+
+
 class FixedIpNotFound(NotFound):
     message = _("No fixed IP associated with id %(id)s.")