diff --git a/static/js/feature_flags.js b/static/js/feature_flags.js
index c9420ef217..9498e2a6d2 100644
--- a/static/js/feature_flags.js
+++ b/static/js/feature_flags.js
@@ -55,7 +55,9 @@ exports.summarize_read_while_narrowed = false;
 exports.collapsible = false;
 
 
-exports.single_message_narrowing = page_params.staging || ('customer12.invalid' === page_params.domain);
+exports.single_message_narrowing = page_params.staging ||
+  _.contains(['customer12.invalid', 'customer10.invalid'], page_params.domain);
+
 return exports;
 
 }());