diff --git a/nova/api/openstack/compute/aggregates.py b/nova/api/openstack/compute/aggregates.py
index bf2ea7f0bf7d..930d65ea87a1 100644
--- a/nova/api/openstack/compute/aggregates.py
+++ b/nova/api/openstack/compute/aggregates.py
@@ -243,7 +243,8 @@ class AggregateController(wsgi.Controller):
                                                            id, metadata)
         except exception.AggregateNotFound as e:
             raise exc.HTTPNotFound(explanation=e.format_message())
-        except exception.InvalidAggregateAction as e:
+        except (exception.InvalidAggregateAction,
+                exception.AggregateMetadataKeyExists) as e:
             raise exc.HTTPBadRequest(explanation=e.format_message())
 
         return self._marshall_aggregate(req, aggregate)
diff --git a/nova/exception.py b/nova/exception.py
index e59b8e3feba0..692047368e31 100644
--- a/nova/exception.py
+++ b/nova/exception.py
@@ -465,6 +465,12 @@ class InvalidAggregateActionUpdateMeta(InvalidAggregateAction):
                 "%(aggregate_id)s. Reason: %(reason)s.")
 
 
+class AggregateMetadataKeyExists(NovaException):
+    msg_fmt = _("Aggregate %(aggregate_id)s already contain metadata "
+                "key %(key)s.")
+    code = 400
+
+
 class InvalidSortKey(Invalid):
     msg_fmt = _("Sort key supplied was not valid.")
 
diff --git a/nova/objects/aggregate.py b/nova/objects/aggregate.py
index 011b990f9e46..48286c588f14 100644
--- a/nova/objects/aggregate.py
+++ b/nova/objects/aggregate.py
@@ -113,8 +113,15 @@ def _metadata_add_to_db(context, aggregate_id, metadata, max_retries=10,
                         api_models.AggregateMetadata.key.in_(all_keys))
                     for meta_ref in query.all():
                         key = meta_ref.key
-                        meta_ref.update({"value": metadata[key]})
-                        already_existing_keys.add(key)
+                        try:
+                            meta_ref.update({"value": metadata[key]})
+                            already_existing_keys.add(key)
+                        except KeyError:
+                            # NOTE(ratailor): When user tries updating
+                            # metadata using case-sensitive key, we get
+                            # KeyError.
+                            raise exception.AggregateMetadataKeyExists(
+                                aggregate_id=aggregate_id, key=key)
 
                 new_entries = []
                 for key, value in metadata.items():
diff --git a/nova/tests/unit/api/openstack/compute/test_aggregates.py b/nova/tests/unit/api/openstack/compute/test_aggregates.py
index 891388590d47..d45bdf1cb66b 100644
--- a/nova/tests/unit/api/openstack/compute/test_aggregates.py
+++ b/nova/tests/unit/api/openstack/compute/test_aggregates.py
@@ -669,6 +669,19 @@ class AggregateTestCaseV21(test.NoDBTestCase):
         self.assertRaises(self.bad_request, eval(self.set_metadata),
                           self.req, "1", body=body)
 
+    def test_set_metadata_case_sensitive(self):
+        body = {"set_metadata": {"metadata": {"foo": "bar"}}}
+        side_effect = exception.AggregateMetadataKeyExists(
+            aggregate_id="2", key="foo")
+
+        with mock.patch.object(self.controller.api,
+                               'update_aggregate_metadata',
+                               side_effect=side_effect) as mock_update:
+            self.assertRaises(exc.HTTPBadRequest, eval(self.set_metadata),
+                              self.req, "2", body=body)
+            mock_update.assert_called_once_with(
+                self.context, "2", body["set_metadata"]["metadata"])
+
     def test_delete_aggregate(self):
         with mock.patch.object(self.controller.api,
                                'delete_aggregate') as mock_del:
diff --git a/nova/tests/unit/objects/test_aggregate.py b/nova/tests/unit/objects/test_aggregate.py
index 3f01c9613dcd..726bff5c166d 100644
--- a/nova/tests/unit/objects/test_aggregate.py
+++ b/nova/tests/unit/objects/test_aggregate.py
@@ -175,6 +175,30 @@ class _TestAggregateObject(object):
                                                       123,
                                                       {'toadd': 'myval'})
 
+    @mock.patch('nova.compute.utils.notify_about_aggregate_action')
+    def test_update_metadata_api_case_sensitive(self, mock_notify):
+        # Mock context.session.query().filter_by().filter().all()
+        fake_context = mock.Mock()
+        mock_query = mock.Mock()
+        fake_context.session.query.return_value = mock_query
+        mock_query = mock_query.filter_by.return_value.filter.return_value
+
+        # Return a fake meta_ref that raises a KeyError
+        fake_meta_ref = mock.Mock()
+        fake_meta_ref.update.side_effect = KeyError
+        mock_query.all.return_value = [fake_meta_ref]
+
+        # Create an aggregate in the database
+        agg = aggregate.Aggregate(
+            context=self.context, name='agg', metadata={'Abc': 'bar'})
+        agg.create()
+
+        # Then replace its context with mock context
+        agg._context = fake_context
+        self.assertRaises(exception.AggregateMetadataKeyExists,
+                          agg.update_metadata,
+                          {'abC': 'barbar'})
+
     @mock.patch('nova.objects.aggregate._host_add_to_db')
     def test_add_host_api(self, mock_host_add_api):
         mock_host_add_api.return_value = {'host': 'bar'}