diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 88aef27..5d85e33 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -129,7 +129,8 @@ public function buildByExtension($extension) {
         //   properly resolve dependencies for all (css) libraries per category,
         //   and only once prior to rendering out an HTML page.
         if ($type == 'css' && !empty($library[$type])) {
-          assert('\Drupal\Core\Asset\LibraryDiscoveryParser::validateCssLibrary($library[$type])', 'CSS must be nested under a category. See https://www.drupal.org/node/2274843.');
+          assert('\Drupal\Core\Asset\LibraryDiscoveryParser::validateCssLibrary($library[$type]) < 2', 'CSS files should be specified as key/value pairs, where the values are configuration options. See https://www.drupal.org/node/2274843.');
+          assert('\Drupal\Core\Asset\LibraryDiscoveryParser::validateCssLibrary($library[$type]) === 0', 'CSS must be nested under a category. See https://www.drupal.org/node/2274843.');
           foreach ($library[$type] as $category => $files) {
             $category_weight = 'CSS_' . strtoupper($category);
             assert('defined($category_weight)', 'Invalid CSS category: ' . $category . '. See https://www.drupal.org/node/2274843.');
@@ -469,23 +470,28 @@ protected function resolveThemeAssetPath($theme_path, $overriding_asset) {
    * @param array $library
    *   The library definition array.
    *
-   * @return bool
-   *   Returns TRUE for a valid library. Proper category checking is not done
-   *   here, as that is captured by a separate assertion.
+   * @return int
+   *   Returns based on validity:
+   *     - 0 if the library definition is valid
+   *     - 1 if the library definition has improper nesting
+   *     - 2 if the library definition specifies files as an array
    */
   public static function validateCssLibrary($library) {
     $categories = [];
     // Verify options first and return early if invalid.
     foreach ($library as $category => $files) {
+      if (!is_array($files)) {
+        return 2;
+      }
       $categories[] = $category;
       foreach ($files as $source => $options) {
         if (!is_array($options)) {
-          return FALSE;
+          return 1;
         }
       }
     }
 
-    return TRUE;
+    return 0;
   }
 
 }
diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
index e18b0ae..efce82f 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
@@ -535,9 +535,6 @@ public function testLibraryWithLicenses() {
 
   /**
    * Verify an assertion fails if CSS declarations have non-existent categories.
-   *
-   * @expectedException \AssertionError
-   * @expectedExceptionMessage Invalid CSS category: bad_category. See https://www.drupal.org/node/2274843.
    */
   public function testCssCategoryAssert() {
     $this->moduleHandler->expects($this->atLeastOnce())
@@ -551,14 +548,12 @@ public function testCssCategoryAssert() {
 
     // This will fail since the CSS declaration isn't properly nested under
     // a category.
+    $this->setExpectedException(\AssertionError::class, 'Invalid CSS category: bad_category. See https://www.drupal.org/node/2274843.');
     $this->libraryDiscoveryParser->buildByExtension('css_bad_category');
   }
 
   /**
    * Verify an assertion fails if CSS declarations aren't properly nested.
-   *
-   * @expectedException \AssertionError
-   * @expectedExceptionMessage CSS must be nested under a category. See https://www.drupal.org/node/2274843.
    */
   public function testCssNestingAssert() {
     $this->moduleHandler->expects($this->atLeastOnce())
@@ -572,9 +567,29 @@ public function testCssNestingAssert() {
 
     // This will fail since the CSS declaration isn't properly nested under
     // a category.
+    $this->setExpectedException(\AssertionError::class, 'CSS must be nested under a category. See https://www.drupal.org/node/2274843.');
     $this->libraryDiscoveryParser->buildByExtension('css_bad_nesting');
   }
 
+  /**
+   * Verify an assertion fails if CSS declarations are a simple array of files.
+   */
+  public function testCssNestingArrayAssert() {
+    $this->moduleHandler->expects($this->atLeastOnce())
+      ->method('moduleExists')
+      ->with('css_bad_nesting_array')
+      ->will($this->returnValue(TRUE));
+
+    $path = __DIR__ . '/library_test_files';
+    $path = substr($path, strlen($this->root) + 1);
+    $this->libraryDiscoveryParser->setPaths('module', 'css_bad_nesting_array', $path);
+
+    // This will fail since the CSS declaration isn't properly nested under
+    // a category.
+    $this->setExpectedException(\AssertionError::class, 'CSS files should be specified as key/value pairs, where the values are configuration options. See https://www.drupal.org/node/2274843.');
+    $this->libraryDiscoveryParser->buildByExtension('css_bad_nesting_array');
+  }
+
 }
 
 /**