diff --git a/roles/web-app-mediawiki/vars/main.yml b/roles/web-app-mediawiki/vars/main.yml
index ae06b8f1..d15c1660 100644
--- a/roles/web-app-mediawiki/vars/main.yml
+++ b/roles/web-app-mediawiki/vars/main.yml
@@ -35,7 +35,7 @@ MEDIAWIKI_OIDC_BUTTON_TEXT:       "{{ OIDC.BUTTON_TEXT }}"
 
 # Extensions
 MEDIAWIKI_EXT_BRANCH:   "REL1_44"
-MEDIAWIKI_EXT_CFG_BASE: "{{ MEDIAWIKI_CONFIG_DIR }}/mwext/{{ MEDIAWIKI_EXT_BRANCH }}"
+MEDIAWIKI_EXT_CFG_BASE: "{{ [ MEDIAWIKI_CONFIG_DIR, 'mwext', MEDIAWIKI_EXT_BRANCH ] | url_join }}"
 MEDIAWIKI_EXT_LIST:
   - name: "PluggableAuth"
     url: "https://codeload.github.com/wikimedia/mediawiki-extensions-PluggableAuth/tar.gz/refs/heads/{{ MEDIAWIKI_EXT_BRANCH }}"