diff --git a/heat/engine/clients/os/nova.py b/heat/engine/clients/os/nova.py
index cd19b2ea50..311eb713f1 100644
--- a/heat/engine/clients/os/nova.py
+++ b/heat/engine/clients/os/nova.py
@@ -47,7 +47,7 @@ CLIENT_NAME = 'nova'
 class NovaClientPlugin(microversion_mixin.MicroversionMixin,
                        client_plugin.ClientPlugin):
 
-    deferred_server_statuses = ['BUILD',
+    deferred_server_statuses = {'BUILD',
                                 'HARD_REBOOT',
                                 'PASSWORD',
                                 'REBOOT',
@@ -56,7 +56,7 @@ class NovaClientPlugin(microversion_mixin.MicroversionMixin,
                                 'REVERT_RESIZE',
                                 'SHUTOFF',
                                 'SUSPENDED',
-                                'VERIFY_RESIZE']
+                                'VERIFY_RESIZE'}
 
     exceptions_module = exceptions
 
diff --git a/heat/engine/resources/aws/ec2/instance.py b/heat/engine/resources/aws/ec2/instance.py
index a829dec051..fc536c29fd 100644
--- a/heat/engine/resources/aws/ec2/instance.py
+++ b/heat/engine/resources/aws/ec2/instance.py
@@ -865,7 +865,7 @@ class Instance(resource.Resource, sh.SchedulerHintsMixin):
         status = cp.get_status(server)
         LOG.debug('%(name)s check_suspend_complete status = %(status)s',
                   {'name': self.name, 'status': status})
-        if status in list(cp.deferred_server_statuses + ['ACTIVE']):
+        if status in (cp.deferred_server_statuses | {'ACTIVE'}):
             return status == 'SUSPENDED'
         else:
             exc = exception.ResourceUnknownStatus(
diff --git a/heat/engine/resources/openstack/nova/server.py b/heat/engine/resources/openstack/nova/server.py
index c6210f8b97..b48fa83345 100644
--- a/heat/engine/resources/openstack/nova/server.py
+++ b/heat/engine/resources/openstack/nova/server.py
@@ -1708,7 +1708,7 @@ class Server(server_base.BaseServer, sh.SchedulerHintsMixin,
         status = cp.get_status(server)
         LOG.debug('%(name)s check_suspend_complete status = %(status)s',
                   {'name': self.name, 'status': status})
-        if status in list(cp.deferred_server_statuses + ['ACTIVE']):
+        if status in (cp.deferred_server_statuses | {'ACTIVE'}):
             return status == 'SUSPENDED'
         else:
             exc = exception.ResourceUnknownStatus(