diff --git a/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php b/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php
index c78fd66..effc7f4 100644
--- a/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php
+++ b/core/lib/Drupal/Core/Asset/CssCollectionRenderer.php
@@ -41,6 +41,9 @@ public function render(array $css_assets) {
     // For filthy IE hack.
     $current_ie_group_keys = NULL;
     $get_ie_group_key = function ($css_asset) {
+      $css_asset += array(
+        'browsers' => NULL,
+      );
       return array($css_asset['type'], $css_asset['preprocess'], $css_asset['group'], $css_asset['every_page'], $css_asset['media'], $css_asset['browsers']);
     };