diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 9acda6a..88aef27 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -129,13 +129,16 @@ 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.');
           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 => $options) {
               if (!isset($options['weight'])) {
                 $options['weight'] = 0;
               }
               // Apply the corresponding weight defined by CSS_* constants.
-              $options['weight'] += constant('CSS_' . strtoupper($category));
+              $options['weight'] += constant($category_weight);
               $library[$type][$source] = $options;
             }
             unset($library[$type][$category]);
@@ -460,4 +463,29 @@ protected function resolveThemeAssetPath($theme_path, $overriding_asset) {
     return $overriding_asset;
   }
 
+  /**
+   * Validates CSS library structure.
+   *
+   * @param array $library
+   *   The library definition array.
+   *
+   * @return bool
+   *   Returns TRUE for a valid library. Proper category checking is not done
+   *   here, as that is captured by a separate assertion.
+   */
+  public static function validateCssLibrary($library) {
+    $categories = [];
+    // Verify options first and return early if invalid.
+    foreach ($library as $category => $files) {
+      $categories[] = $category;
+      foreach ($files as $source => $options) {
+        if (!is_array($options)) {
+          return FALSE;
+        }
+      }
+    }
+
+    return TRUE;
+  }
+
 }
diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
index ed0c9ce..e18b0ae 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
@@ -533,6 +533,48 @@ public function testLibraryWithLicenses() {
     $this->assertEquals($library['license'], $expected_license);
   }
 
+  /**
+   * 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())
+      ->method('moduleExists')
+      ->with('css_bad_category')
+      ->will($this->returnValue(TRUE));
+
+    $path = __DIR__ . '/library_test_files';
+    $path = substr($path, strlen($this->root) + 1);
+    $this->libraryDiscoveryParser->setPaths('module', 'css_bad_category', $path);
+
+    // This will fail since the CSS declaration isn't properly nested under
+    // a category.
+    $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
new file mode 100644
index 0000000..803efc3
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_category.libraries.yml
@@ -0,0 +1,5 @@
+bad_category:
+  css:
+    # 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 }
diff --git a/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml b/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml
index 9a8b4dd..068a9a3 100644
--- a/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml
+++ b/core/tests/Drupal/Tests/Core/Asset/library_test_files/dependencies.libraries.yml
@@ -1,6 +1,7 @@
 example:
   css:
-    css/example.js: {}
+    theme:
+      css/example.js: {}
   dependencies:
     - external/example_external
     - example_module/example