diff --git a/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php b/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php
index 527dd5c..c78fd66 100644
--- a/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php
+++ b/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php
@@ -48,6 +48,11 @@ public function render(array $css_assets) {
     $css_assets_keys = array_keys($css_assets);
     for ($i = 0; $i < count($css_assets_keys); $i++) {
       $css_asset = $css_assets[$css_assets_keys[$i]];
+      if (is_array($css_asset)) {
+        $css_asset += array(
+          'browsers' => NULL
+        );
+      }
       switch ($css_asset['type']) {
         // For file items, there are three possibilites.
         // - There are up to 31 CSS assets on the page (some of which may be