diff --git a/mattermost/hooks/web_proxy-relation-joined b/mattermost/hooks/web_proxy-relation-joined
index 7ab7770..a6ed824 100755
--- a/mattermost/hooks/web_proxy-relation-joined
+++ b/mattermost/hooks/web_proxy-relation-joined
@@ -8,5 +8,5 @@ config-add "\
 services:
   $MASTER_BASE_SERVICE_NAME:
     environment:
-      MM_SERVICESETTINGS_SITEURL=${URL}
+      MM_SERVICESETTINGS_SITEURL: ${URL}
 "