diff --git a/heatclient/v1/shell.py b/heatclient/v1/shell.py
index cd17970b..2d5929b2 100644
--- a/heatclient/v1/shell.py
+++ b/heatclient/v1/shell.py
@@ -205,7 +205,7 @@ def do_stack_preview(hc, args):
         'disable_rollback': not(args.enable_rollback),
         'parameters': utils.format_parameters(args.parameters),
         'template': template,
-        'files': dict(tpl_files.items() + env_files.items()),
+        'files': dict(list(tpl_files.items()) + list(env_files.items())),
         'environment': env
     }