diff --git a/ops/model.py b/ops/model.py
index 9974af9..e627951 100644
--- a/ops/model.py
+++ b/ops/model.py
@@ -1194,6 +1194,7 @@ class Container:
         If no service names are specified, return status information for all
         services, otherwise return information for only the given services.
         """
+        service_names = service_names or None
         services = self._pebble.get_services(service_names)
         return ServiceInfoMapping(services)
 

diff --git a/test/test_model.py b/test/test_model.py
index c8e890f..5004d88 100755
--- a/test/test_model.py
+++ b/test/test_model.py
@@ -772,9 +772,26 @@ class TestApplication(unittest.TestCase):
             resources:
               foo: {type: file, filename: foo.txt}
               bar: {type: file, filename: bar.txt}
+            containers:
+              bar:
+                k: v
         ''')
         self.peer_rel_id = self.harness.add_relation('db2', 'db2')
         self.app = self.harness.model.app
+        self.addCleanup(self.harness.cleanup)
+
+    # Tests fix for https://github.com/canonical/operator/issues/694.
+    def test_mocked_get_services(self):
+        self.harness.begin()
+        c = self.harness.charm.unit.get_container('bar')
+        c.add_layer('layer1', {
+            'summary': 'layer',
+            'services': {"baz": {'override': 'replace', 'summary': 'echo', 'command': 'echo 1'}},
+        })
+
+        s = c.get_service('baz')  # So far, so good
+        self.assertTrue(s)
+        self.assertTrue('baz' in c.get_services())
 
     def test_planned_units(self):
         rel_id = self.peer_rel_id
@@ -1062,7 +1079,7 @@ containers:
         self.assertEqual(services['s2'].current, ops.pebble.ServiceStatus.INACTIVE)
 
         self.assertEqual(self.pebble.requests, [
-            ('get_services', ()),
+            ('get_services', None),
             ('get_services', ('s1', 's2')),
         ])
 

Harness.Container.get_services() does not seem correctly mocked
I think this test should work, but it does not with `ops 1.3.0`:

```
    def test_mocked_get_services(self):
        self.harness.begin()
        traefik_container = self.harness.charm.unit.get_container("traefik")
        traefik_container.add_layer("traefik_layer", {
            "summary": "Traefik layer",
            "description": "Pebble config layer for Traefik",
            "services": {
                "traefik": {
                    "override": "replace",
                    "summary": "Traefik",
                    "command": "/usr/bin/traefik",
                },
            },
        })

        assert traefik_container.get_service("traefik")  # So far, so good
        assert "traefik" in traefik_container.get_services()  # FAILS, `traefik_container.get_services()` returns {} 
```
test/test_model.py::TestApplication::test_mocked_get_services test/test_model.py::TestContainerPebble::test_get_services
test/test_model.py::TestModel::test_active_message_default test/test_model.py::TestModel::test_app_immutable test/test_model.py::TestModel::test_app_relation_data_modify_local_as_leader test/test_model.py::TestModel::test_app_relation_data_modify_local_as_minion test/test_model.py::TestModel::test_base_status_instance_raises test/test_model.py::TestModel::test_config test/test_model.py::TestModel::test_config_immutable test/test_model.py::TestModel::test_get_relation test/test_model.py::TestModel::test_is_leader test/test_model.py::TestModel::test_local_set_valid_app_status test/test_model.py::TestModel::test_local_set_valid_unit_status test/test_model.py::TestModel::test_model_attributes test/test_model.py::TestModel::test_model_name_from_backend test/test_model.py::TestModel::test_our_unit_is_our test/test_model.py::TestModel::test_peer_relation_app test/test_model.py::TestModel::test_pod_immutable test/test_model.py::TestModel::test_pod_spec test/test_model.py::TestModel::test_relation_data_del_key test/test_model.py::TestModel::test_relation_data_del_missing_key test/test_model.py::TestModel::test_relation_data_modify_our test/test_model.py::TestModel::test_relation_data_modify_remote test/test_model.py::TestModel::test_relation_data_type_check test/test_model.py::TestModel::test_relation_no_units test/test_model.py::TestModel::test_relation_set_fail test/test_model.py::TestModel::test_relations_immutable test/test_model.py::TestModel::test_relations_keys test/test_model.py::TestModel::test_remote_app_relation_data test/test_model.py::TestModel::test_remote_app_status test/test_model.py::TestModel::test_remote_unit_status test/test_model.py::TestModel::test_remote_units_is_our test/test_model.py::TestModel::test_resources test/test_model.py::TestModel::test_resources_immutable test/test_model.py::TestModel::test_set_app_status_invalid test/test_model.py::TestModel::test_set_app_status_non_leader_raises test/test_model.py::TestModel::test_set_unit_status_invalid test/test_model.py::TestModel::test_status_eq test/test_model.py::TestModel::test_status_repr test/test_model.py::TestModel::test_storage test/test_model.py::TestModel::test_storages_immutable test/test_model.py::TestModel::test_unit_immutable test/test_model.py::TestModel::test_unit_relation_data test/test_model.py::TestModel::test_workload_version test/test_model.py::TestModel::test_workload_version_invalid test/test_model.py::TestApplication::test_planned_units test/test_model.py::TestApplication::test_planned_units_garbage_values test/test_model.py::TestApplication::test_planned_units_override test/test_model.py::TestApplication::test_planned_units_user_set test/test_model.py::TestContainers::test_unit_containers test/test_model.py::TestContainers::test_unit_get_container test/test_model.py::TestContainerPebble::test_add_layer test/test_model.py::TestContainerPebble::test_autostart test/test_model.py::TestContainerPebble::test_bare_can_connect_call test/test_model.py::TestContainerPebble::test_exec test/test_model.py::TestContainerPebble::test_get_plan test/test_model.py::TestContainerPebble::test_get_service test/test_model.py::TestContainerPebble::test_get_system_info test/test_model.py::TestContainerPebble::test_list_files test/test_model.py::TestContainerPebble::test_make_dir test/test_model.py::TestContainerPebble::test_pull test/test_model.py::TestContainerPebble::test_push test/test_model.py::TestContainerPebble::test_remove_path test/test_model.py::TestContainerPebble::test_replan test/test_model.py::TestContainerPebble::test_restart test/test_model.py::TestContainerPebble::test_restart_fallback test/test_model.py::TestContainerPebble::test_restart_fallback_non_400_error test/test_model.py::TestContainerPebble::test_restart_no_arguments test/test_model.py::TestContainerPebble::test_send_signal test/test_model.py::TestContainerPebble::test_socket_path test/test_model.py::TestContainerPebble::test_start test/test_model.py::TestContainerPebble::test_start_no_arguments test/test_model.py::TestContainerPebble::test_stop test/test_model.py::TestContainerPebble::test_stop_no_arguments test/test_model.py::TestContainerPebble::test_type_errors test/test_model.py::TestModelBindings::test_binding_by_relation test/test_model.py::TestModelBindings::test_binding_by_relation_name test/test_model.py::TestModelBindings::test_binding_no_iface_name test/test_model.py::TestModelBindings::test_dead_relations test/test_model.py::TestModelBindings::test_empty_bind_addresses test/test_model.py::TestModelBindings::test_empty_interface_info test/test_model.py::TestModelBindings::test_invalid_keys test/test_model.py::TestModelBindings::test_missing_bind_addresses test/test_model.py::TestModelBindings::test_missing_egress_subnets test/test_model.py::TestModelBindings::test_missing_ingress_addresses test/test_model.py::TestModelBindings::test_no_bind_addresses test/test_model.py::TestModelBackend::test_action_fail test/test_model.py::TestModelBackend::test_action_get test/test_model.py::TestModelBackend::test_action_get_error test/test_model.py::TestModelBackend::test_action_log test/test_model.py::TestModelBackend::test_action_log_error test/test_model.py::TestModelBackend::test_action_set test/test_model.py::TestModelBackend::test_action_set_dotted_dict test/test_model.py::TestModelBackend::test_action_set_duplicated_keys test/test_model.py::TestModelBackend::test_action_set_error test/test_model.py::TestModelBackend::test_action_set_key_validation test/test_model.py::TestModelBackend::test_action_set_more_nested test/test_model.py::TestModelBackend::test_action_set_nested test/test_model.py::TestModelBackend::test_application_version_set test/test_model.py::TestModelBackend::test_application_version_set_invalid test/test_model.py::TestModelBackend::test_invalid_metric_label_values test/test_model.py::TestModelBackend::test_invalid_metric_labels test/test_model.py::TestModelBackend::test_invalid_metric_names test/test_model.py::TestModelBackend::test_invalid_metric_values test/test_model.py::TestModelBackend::test_is_leader_refresh test/test_model.py::TestModelBackend::test_juju_log test/test_model.py::TestModelBackend::test_local_set_invalid_status test/test_model.py::TestModelBackend::test_network_get test/test_model.py::TestModelBackend::test_network_get_errors test/test_model.py::TestModelBackend::test_relation_get_juju_version_quirks test/test_model.py::TestModelBackend::test_relation_get_set_is_app_arg test/test_model.py::TestModelBackend::test_relation_remote_app_name_env test/test_model.py::TestModelBackend::test_relation_remote_app_name_script_errors test/test_model.py::TestModelBackend::test_relation_remote_app_name_script_success test/test_model.py::TestModelBackend::test_relation_set_juju_version_quirks test/test_model.py::TestModelBackend::test_relation_tool_errors test/test_model.py::TestModelBackend::test_status_get test/test_model.py::TestModelBackend::test_status_is_app_forced_kwargs test/test_model.py::TestModelBackend::test_status_set_is_app_not_bool_raises test/test_model.py::TestModelBackend::test_storage_tool_errors test/test_model.py::TestModelBackend::test_valid_metrics test/test_model.py::TestLazyMapping::test_invalidate