diff --git a/nova/api/openstack/compute/server_diagnostics.py b/nova/api/openstack/compute/server_diagnostics.py
index 284f6ad462de..49bfd598ce6e 100644
--- a/nova/api/openstack/compute/server_diagnostics.py
+++ b/nova/api/openstack/compute/server_diagnostics.py
@@ -13,6 +13,8 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import webob
+
 from nova.api.openstack import common
 from nova.api.openstack import extensions
 from nova.api.openstack import wsgi
@@ -45,6 +47,8 @@ class ServerDiagnosticsController(wsgi.Controller):
         except exception.InstanceInvalidState as state_error:
             common.raise_http_conflict_for_instance_invalid_state(state_error,
                     'get_diagnostics', server_id)
+        except exception.InstanceNotReady as e:
+            raise webob.exc.HTTPConflict(explanation=e.format_message())
         except NotImplementedError:
             common.raise_feature_not_supported()
 
diff --git a/nova/compute/api.py b/nova/compute/api.py
index a278ba836b0c..484156d0e5c5 100644
--- a/nova/compute/api.py
+++ b/nova/compute/api.py
@@ -2731,10 +2731,12 @@ class API(base.Base):
         self._record_action_start(context, instance, instance_actions.UNPAUSE)
         self.compute_rpcapi.unpause_instance(context, instance)
 
+    @check_instance_host
     def get_diagnostics(self, context, instance):
         """Retrieve diagnostics for the given instance."""
         return self.compute_rpcapi.get_diagnostics(context, instance=instance)
 
+    @check_instance_host
     def get_instance_diagnostics(self, context, instance):
         """Retrieve diagnostics for the given instance."""
         return self.compute_rpcapi.get_instance_diagnostics(context,
diff --git a/nova/tests/unit/api/openstack/compute/test_server_diagnostics.py b/nova/tests/unit/api/openstack/compute/test_server_diagnostics.py
index 8423b96cac74..33e9c539936b 100644
--- a/nova/tests/unit/api/openstack/compute/test_server_diagnostics.py
+++ b/nova/tests/unit/api/openstack/compute/test_server_diagnostics.py
@@ -79,6 +79,15 @@ class ServerDiagnosticsTestV21(test.NoDBTestCase):
         res = req.get_response(self.router)
         self.assertEqual(409, res.status_int)
 
+    @mock.patch.object(compute_api.API, 'get_diagnostics',
+                       side_effect=exception.InstanceNotReady('fake message'))
+    @mock.patch.object(compute_api.API, 'get', fake_instance_get)
+    def test_get_diagnostics_raise_instance_not_ready(self,
+                                                      mock_get_diagnostics):
+        req = self._get_request()
+        res = req.get_response(self.router)
+        self.assertEqual(409, res.status_int)
+
     @mock.patch.object(compute_api.API, 'get_diagnostics',
                 side_effect=NotImplementedError)
     @mock.patch.object(compute_api.API, 'get', fake_instance_get)
diff --git a/nova/tests/unit/compute/test_compute_api.py b/nova/tests/unit/compute/test_compute_api.py
index c9ba95c78844..9b15b5e49a30 100644
--- a/nova/tests/unit/compute/test_compute_api.py
+++ b/nova/tests/unit/compute/test_compute_api.py
@@ -1941,6 +1941,20 @@ class _ComputeAPIUnitTestMixIn(object):
         self.assertEqual(vm_states.PAUSED, instance.vm_state)
         self.assertEqual(task_states.UNPAUSING, instance.task_state)
 
+    def test_get_diagnostics_none_host(self):
+        instance = self._create_instance_obj()
+        instance.host = None
+        self.assertRaises(exception.InstanceNotReady,
+                          self.compute_api.get_diagnostics,
+                          self.context, instance)
+
+    def test_get_instance_diagnostics_none_host(self):
+        instance = self._create_instance_obj()
+        instance.host = None
+        self.assertRaises(exception.InstanceNotReady,
+                          self.compute_api.get_instance_diagnostics,
+                          self.context, instance)
+
     def test_live_migrate_active_vm_state(self):
         instance = self._create_instance_obj()
         self._live_migrate_instance(instance)