diff --git a/API_REFERENCE.rst b/API_REFERENCE.rst
index d45352e..0f763c4 100644
--- a/API_REFERENCE.rst
+++ b/API_REFERENCE.rst
@@ -71,11 +71,11 @@ TapFlow Represents the port from which the traffic needs to be mirrored.
                         'validate': {'type:uuid': None},
                         'required_by_policy': True, 'is_visible': True},
         'direction': {'allow_post': True, 'allow_put': False,
-                      'validate': {'type:string': direction_enum},
+                      'validate': {'type:values': direction_enum},
                       'is_visible': True}
     }
 
-    direction_enum = [None, 'IN', 'OUT', 'BOTH']
+    direction_enum = ['IN', 'OUT', 'BOTH']
 
 
 Multiple TapFlow instances can be associated with a single TapService
diff --git a/neutron_taas/extensions/taas.py b/neutron_taas/extensions/taas.py
index e2bafb4..ffe64ba 100644
--- a/neutron_taas/extensions/taas.py
+++ b/neutron_taas/extensions/taas.py
@@ -61,7 +61,7 @@ class TapServiceLimitReached(qexception.OverQuota):
     message = _("Reached the maximum quota for Tap Services")
 
 
-direction_enum = [None, 'IN', 'OUT', 'BOTH']
+direction_enum = ['IN', 'OUT', 'BOTH']
 
 
 '''
@@ -117,7 +117,7 @@ RESOURCE_ATTRIBUTE_MAP = {
                         'validate': {'type:uuid': None},
                         'required_by_policy': True, 'is_visible': True},
         'direction': {'allow_post': True, 'allow_put': False,
-                      'validate': {'type:string': direction_enum},
+                      'validate': {'type:values': direction_enum},
                       'is_visible': True}
     }
 }