diff --git a/vars/main.yml b/vars/main.yml
index 3b1fe338..bc2522b5 100644
--- a/vars/main.yml
+++ b/vars/main.yml
@@ -160,8 +160,8 @@ _tempest_test_gathered_excludelist: "{{ query('vars', *query('varnames', '^' ~ t
 
 # maintain backward compatibility by combining the original role default
 # with any dynamically gathered variables
-_tempest_test_includelist: "{{ tempest_test_includelist | union(_tempest_test_gathered_includelist) }}"
-_tempest_test_excludelist: "{{ tempest_test_excludelist | union(_tempest_test_gathered_excludelist) }}"
+_tempest_test_includelist: "{{ tempest_test_includelist + _tempest_test_gathered_includelist }}"
+_tempest_test_excludelist: "{{ tempest_test_excludelist + _tempest_test_gathered_excludelist }}"
 
 # gather tempest config overrides from any variables starting wth the defined search pattern
 _tempest_tempest_conf_overrides_gathered: "{{ query('vars', *query('varnames', '^' ~ tempest_tempest_conf_search_overrides_pattern)) | combine }}"