diff --git a/app/code/Magento/Deploy/Package/Processor/PreProcessor/Css.php b/app/code/Magento/Deploy/Package/Processor/PreProcessor/Css.php
index b3461e3ab4a22..fd06e20c5c7d7 100644
--- a/app/code/Magento/Deploy/Package/Processor/PreProcessor/Css.php
+++ b/app/code/Magento/Deploy/Package/Processor/PreProcessor/Css.php
@@ -3,6 +3,7 @@
  * Copyright © Magento, Inc. All rights reserved.
  * See COPYING.txt for license details.
  */
+
 namespace Magento\Deploy\Package\Processor\PreProcessor;
 
 use Magento\Deploy\Console\DeployStaticOptions;
@@ -92,8 +93,7 @@ public function process(Package $package, array $options)
     }
 
     /**
-     * Checks if there are imports of CSS files or images within the given CSS file
-     * which exists in the current package
+     * Checks if there are imports of CSS files or images within the given CSS file which exists in the current package.
      *
      * @param PackageFile $parentFile
      * @param Package $package
@@ -180,11 +180,13 @@ private function buildMap($packagePath, $filePath, $fullPath)
      */
     private function collectFileMap($fileName)
     {
-        $result = isset($this->map[$fileName]) ? $this->map[$fileName] : [];
+        $result = [$this->map[$fileName] ?? []];
+
         foreach ($result as $path) {
-            $result = array_merge($result, $this->collectFileMap($path));
+            $result[] = $this->collectFileMap($path);
         }
-        return array_unique($result);
+
+        return array_unique(array_merge(...$result));
     }
 
     /**