diff --git a/nginx/conf.d/default.conf b/nginx/conf.d/default.conf
index 3633cc2..aeda057 100644
--- a/nginx/conf.d/default.conf
+++ b/nginx/conf.d/default.conf
@@ -57,7 +57,12 @@ server {
 
   location / {
     set $ui_upstream "ui";
-    proxy_pass http://$ui_upstream:3000/;
+    # No trailing-slash URI: with a VARIABLE upstream, a trailing "/" makes
+    # nginx send "/" to the UI for every sub-path (assets, /i/<invoice>), so
+    # the app's index HTML is returned for /_next/static/*.css (200 text/html)
+    # → the whole UI renders unstyled and client routing dies. Omit the URI so
+    # the full request path is preserved, exactly like the /api/ block above.
+    proxy_pass http://$ui_upstream:3000;
     proxy_http_version 1.1;
     proxy_set_header Host $host;
     proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;