diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 9acda6a..3eaf553 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -130,12 +130,14 @@ 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 => $options) {
               if (!isset($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]);
diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
index ed0c9ce..c977d39 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
@@ -533,6 +533,27 @@ public function testLibraryWithLicenses() {
     $this->assertEquals($library['license'], $expected_license);
   }
 
+  /**
+   * Verify an assertion fails if CSS declarations aren't properly nested.
+   *
+   * @expectedException \AssertionError
+   * @expectedExceptionMessage Invalid CSS category: css/styles.css. 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');
+  }
+
 }
 
 /**
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..3b06367
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Asset/library_test_files/css_bad_category.libraries.yml
@@ -0,0 +1,4 @@
+bad_category:
+  css:
+    # No nesting here will break.
+    css/styles.css: {  }
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