diff --git a/core/lib/Drupal/Core/Asset/CssOptimizer.php b/core/lib/Drupal/Core/Asset/CssOptimizer.php
index ab0f2f9..858cc5f 100644
--- a/core/lib/Drupal/Core/Asset/CssOptimizer.php
+++ b/core/lib/Drupal/Core/Asset/CssOptimizer.php
@@ -61,6 +61,8 @@ public function clean($contents) {
   protected function processFile($css_asset) {
     $contents = $this->loadFile($css_asset['data'], TRUE);
 
+    $contents = $this->clean($contents);
+
     // Get the parent directory of this file, relative to the Drupal root.
     $css_base_path = substr($css_asset['data'], 0, strrpos($css_asset['data'], '/'));
     // Store base path.
@@ -177,7 +179,7 @@ protected function loadNestedFile($matches) {
    */
   protected function processCss($contents, $optimize = FALSE) {
     // Remove unwanted CSS code that cause issues.
-    $this->clean($contents);
+    $contents = $this->clean($contents);
 
     if ($optimize) {
       // Perform some safe CSS optimizations.
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_newline.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_newline.css
index 68f3f42..21aacf2 100644
--- a/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_newline.css
+++ b/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_newline.css
@@ -1,2 +1,2 @@
-@charset 'UTF-8';
+@charset "UTF-8";
 html{font-family:"sans-serif";}
diff --git a/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_sameline.css.optimized.css b/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_sameline.css.optimized.css
deleted file mode 100644
index c9e6ade..0000000
--- a/core/tests/Drupal/Tests/Core/Asset/css_test_files/charset_sameline.css.optimized.css
+++ /dev/null
@@ -1 +0,0 @@
-html{font-family:"sans-serif";}