diff --git a/nova/virt/zun/driver.py b/nova/virt/zun/driver.py
index a7c97dc95..47eaaafc9 100644
--- a/nova/virt/zun/driver.py
+++ b/nova/virt/zun/driver.py
@@ -234,8 +234,8 @@ class DockerDriver(driver.ComputeDriver):
         self._stop_firewall(instance, network_info)
 
     def _encode_utf8(self, value):
-        if six.PY2 and not isinstance(value, unicode):
-            value = unicode(value)
+        if six.PY2 and not isinstance(value, six.text_type):
+            value = six.text_type(value)
         return value.encode('utf-8')
 
     def _find_container_by_instance(self, instance):