diff --git a/library/Icinga/Web/JavaScript.php b/library/Icinga/Web/JavaScript.php
index 1865136c76..1c09a1e680 100644
--- a/library/Icinga/Web/JavaScript.php
+++ b/library/Icinga/Web/JavaScript.php
@@ -238,8 +238,9 @@ public static function optimizeDefine($js, $filePath, $basePath, $packageName)
         try {
             $dependencies = $match[2] ? Json::decode($match[2]) : [];
             foreach ($dependencies as &$dependencyName) {
-                if ($dependencyName === 'exports') {
+                if ($dependencyName === 'exports' || str_starts_with($dependencyName, 'icinga/legacy-app/')) {
                     // exports is a special keyword and doesn't need optimization
+                    // 'app/legacy-app/...' is a custom-defined path and doesn’t need optimization either.
                     continue;
                 }
 
diff --git a/public/js/icinga.js b/public/js/icinga.js
index a0e060ecfb..634fc9a52c 100644
--- a/public/js/icinga.js
+++ b/public/js/icinga.js
@@ -100,6 +100,8 @@
             this.events     = new Icinga.Events(this);
             this.history    = new Icinga.History(this);
 
+            define("icinga/legacy-app/Icinga", [], () => Icinga);
+
             // Initialize all available behaviors
             for (const name in Icinga.Behaviors) {
                 const behavior = new Icinga.Behaviors[name](this);