diff --git a/src/Asset/AmpCssCollectionRenderer.php b/src/Asset/AmpCssCollectionRenderer.php
index 0c91e1d..eed21c9 100644
--- a/src/Asset/AmpCssCollectionRenderer.php
+++ b/src/Asset/AmpCssCollectionRenderer.php
@@ -127,6 +127,10 @@ class AmpCssCollectionRenderer extends CssCollectionRenderer {
         $urls = preg_match_all('/@import url\("(.+)\?/', $element['#value'], $matches);
         $all_css = [];
         foreach ($matches[1] as $url) {
+          // Strip virtual subfolders from url
+          if (\Drupal::request()->getBasePath() !== '/') {
+            $url = str_replace(\Drupal::request()->getBasePath(), '', $url);
+          }
           // Skip empty and missing files.
           if ($css = @file_get_contents(DRUPAL_ROOT . $url)) {
             if ($needs_rewrite) {
@@ -159,6 +163,10 @@ class AmpCssCollectionRenderer extends CssCollectionRenderer {
           }
         }
         else {
+          // Strip virtual subfolders from url
+          if (\Drupal::request()->getBasePath() !== '/') {
+            $url = str_replace(\Drupal::request()->getBasePath(), '', $url);
+          }
           // Strip any querystring off the url.
           if (strpos($url, '?') !== FALSE) {
             list($url, $query) = explode('?', $url);