diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 9798bc9..b07937a 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -132,13 +132,16 @@ public function buildByExtension($extension) {
         //   and only once prior to rendering out an HTML page.
         if ($type == 'css' && !empty($library[$type])) {
           foreach ($library[$type] as $category => $files) {
-            // Allow empty categories.
-           if (defined('CSS_' . strtoupper($category)) && empty($files)) {
-              unset($library[$type][$category]);
-              continue;
-            }
             if (empty($files)) {
-              throw new InvalidCssDefinitionException(sprintf("Every CSS asset must be nested under a SMACSS category in the %s.library.yml file ('%s' seems to be a file name, allowed categories are Base, Layout, Component, State, Theme)", $extension, $category));
+              // Allow empty categories.
+              if (defined('CSS_' . strtoupper($category))) {
+                unset($library[$type][$category]);
+                continue;
+              }
+              // Files are are in $category and $files is empty.
+              else if (empty($files)) {
+                throw new InvalidCssDefinitionException(sprintf("Every CSS asset must be nested under a SMACSS category in the %s.library.yml file ('%s' seems to be a file name, allowed categories are Base, Layout, Component, State, Theme)", $extension, $category));
+              }
             }
             foreach ($files as $source => $options) {
               if (!isset($options['weight'])) {