diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 3eaf553..54eed4f 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -130,9 +130,10 @@ 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) {
-            $category_weight = 'CSS_' . strtoupper($category);
-            assert('defined($category_weight)', 'Invalid CSS category: ' . $category . '. See https://www.drupal.org/node/2274843.');
             foreach ($files as $source => $options) {
+              assert('is_array($options)', 'CSS must be nested under a category. See https://www.drupal.org/node/2274843.');
+              $category_weight = 'CSS_' . strtoupper($category);
+              assert('defined($category_weight)', 'Invalid CSS category: ' . $category . '. See https://www.drupal.org/node/2274843.');
               if (!isset($options['weight'])) {
                 $options['weight'] = 0;
               }
diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
index c977d39..e18b0ae 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
@@ -534,10 +534,10 @@ public function testLibraryWithLicenses() {
   }
 
   /**
-   * Verify an assertion fails if CSS declarations aren't properly nested.
+   * Verify an assertion fails if CSS declarations have non-existent categories.
    *
    * @expectedException \AssertionError
-   * @expectedExceptionMessage Invalid CSS category: css/styles.css. See https://www.drupal.org/node/2274843.
+   * @expectedExceptionMessage Invalid CSS category: bad_category. See https://www.drupal.org/node/2274843.
    */
   public function testCssCategoryAssert() {
     $this->moduleHandler->expects($this->atLeastOnce())
@@ -554,6 +554,27 @@ public function testCssCategoryAssert() {
     $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())
+      ->method('moduleExists')
+      ->with('css_bad_nesting')
+      ->will($this->returnValue(TRUE));
+
+    $path = __DIR__ . '/library_test_files';
+    $path = substr($path, strlen($this->root) + 1);
+    $this->libraryDiscoveryParser->setPaths('module', 'css_bad_nesting', $path);
+
+    // This will fail since the CSS declaration isn't properly nested under
+    // a category.
+    $this->libraryDiscoveryParser->buildByExtension('css_bad_nesting');
+  }
+
 }
 
 /**
diff --git a/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_category.libraries.yml b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_category.libraries.yml
index 5a76a30..803efc3 100644
--- a/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_category.libraries.yml
+++ b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_category.libraries.yml
@@ -1,4 +1,5 @@
 bad_category:
   css:
-    # No nesting here will break.
-    css/styles.css: { minified: true }
+    # Non-existent category.
+    bad_category:
+      css/styles.css: { minified: true }
diff --git a/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_nesting.libraries.yml b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_nesting.libraries.yml
new file mode 100644
index 0000000..44c9c0e
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_nesting.libraries.yml
@@ -0,0 +1,4 @@
+bad_nesting:
+  css:
+    # No nesting here will break.
+    css/styles.css: { minified: true }