diff --git a/nova/api/openstack/compute/contrib/floating_ips.py b/nova/api/openstack/compute/contrib/floating_ips.py
index 2f3c5619916d..bf1246ccba73 100644
--- a/nova/api/openstack/compute/contrib/floating_ips.py
+++ b/nova/api/openstack/compute/contrib/floating_ips.py
@@ -294,7 +294,9 @@ class FloatingIPActionController(wsgi.Controller):
             disassociate_floating_ip(self, context, instance, address)
             return webob.Response(status_int=202)
         else:
-            return webob.Response(status_int=404)
+            msg = _("Floating ip %(address)s is not associated with instance "
+                    "%(id)s.") % locals()
+            raise webob.exc.HTTPUnprocessableEntity(explanation=msg)
 
 
 class Floating_ips(extensions.ExtensionDescriptor):
diff --git a/nova/tests/api/openstack/compute/contrib/test_floating_ips.py b/nova/tests/api/openstack/compute/contrib/test_floating_ips.py
index 864ab7a9fe57..00759a7ef613 100644
--- a/nova/tests/api/openstack/compute/contrib/test_floating_ips.py
+++ b/nova/tests/api/openstack/compute/contrib/test_floating_ips.py
@@ -364,8 +364,9 @@ class FloatingIpTest(test.TestCase):
         body = dict(removeFloatingIp=dict(address='10.10.10.10'))
 
         req = fakes.HTTPRequest.blank('/v2/fake/servers/test_inst/action')
-        rsp = self.manager._remove_floating_ip(req, 'test_inst', body)
-        self.assertTrue(rsp.status_int == 404)
+        self.assertRaises(webob.exc.HTTPUnprocessableEntity,
+                          self.manager._remove_floating_ip,
+                          req, 'test_inst', body)
 
     def test_floating_ip_associate_non_existent_ip(self):
         def fake_network_api_associate(self, context, instance,
@@ -414,8 +415,9 @@ class FloatingIpTest(test.TestCase):
         body = dict(removeFloatingIp=dict(address='10.10.10.10'))
 
         req = fakes.HTTPRequest.blank('/v2/fake/servers/test_inst/action')
-        rsp = self.manager._remove_floating_ip(req, wrong_uuid, body)
-        self.assertTrue(rsp.status_int == 404)
+        self.assertRaises(webob.exc.HTTPUnprocessableEntity,
+                          self.manager._remove_floating_ip,
+                          req, wrong_uuid, body)
 
     def test_floating_ip_disassociate_wrong_instance_id(self):
         def get_instance_by_floating_ip_addr(self, context, address):
@@ -428,8 +430,9 @@ class FloatingIpTest(test.TestCase):
         body = dict(removeFloatingIp=dict(address='10.10.10.10'))
 
         req = fakes.HTTPRequest.blank('/v2/fake/servers/test_inst/action')
-        rsp = self.manager._remove_floating_ip(req, 'test_inst', body)
-        self.assertTrue(rsp.status_int == 404)
+        self.assertRaises(webob.exc.HTTPUnprocessableEntity,
+                          self.manager._remove_floating_ip,
+                          req, 'test_inst', body)
 
     def test_floating_ip_disassociate_auto_assigned(self):
         def fake_get_floating_ip_addr_auto_assigned(self, context, address):