diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index b3354dc..b4afdb5 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -124,7 +124,7 @@ public function buildByExtension($extension) {
         if ($type == 'css' && !empty($library[$type])) {
           foreach ($library[$type] as $category => $files) {
             if (empty($files)) {
-              throw new InvalidCssDefinitionException(sprintf("You have to key the css files in the %s.library.yml file by category ('%s' seems to be a file name, allowed categories are Base, Layout, Component, State, Theme)", $extension, $category));
+              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'])) {
@@ -136,7 +136,7 @@ public function buildByExtension($extension) {
                 $options['weight'] += constant('CSS_' . strtoupper($category));
               }
               else {
-                throw new InvalidCssDefinitionException(sprintf("You have used an invalid category %s in %s.libraries.yml. Allowed categories are Base, Layout, Component, State, Theme)", $category, $extension));
+                throw new InvalidCssDefinitionException(sprintf("You must use a valid SMACSS category to nest a CSS asset. %s is invalid in %s.libraries.yml. Allowed categories are Base, Layout, Component, State, Theme)", $category, $extension));
               }
               $library[$type][$source] = $options;
             }
diff --git a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
index c73cc35..6c92672 100644
--- a/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
+++ b/core/tests/Drupal/Tests/Core/Asset/LibraryDiscoveryParserTest.php
@@ -182,7 +182,8 @@ public function testBuildByExtensionWithMissingInformation() {
    * Tests that an exception is thrown when no CSS category is specified.
    *
    * @expectedException \Drupal\Core\Asset\Exception\InvalidCssDefinitionException
-   * @expectedExceptionMessage Every CSS asset must be nested under a SMACSS category, i.e. either theme, or base. See for details.
+   * @expectedExceptionMessage Every CSS asset must be nested under a SMACSS category
+   * @expectedExceptionMessage css_missing_category
    *
    * @covers ::buildByExtension
    */
@@ -203,7 +204,8 @@ public function testBuildByExtensionWithMissingCSSCategory() {
    * Tests that an exception is thrown when no an invalid CSS category is used.
    *
    * @expectedException \Drupal\Core\Asset\Exception\InvalidCssDefinitionException
-   * @expectedExceptionMessage You must use a valid SMACSS category to nest a CSS asset, i.e. either theme, or base. See for details.
+   * @expectedExceptionMessage You must use a valid SMACSS category to nest a CSS asset
+   * @expectedExceptionMessage css_invalid_category
    *
    * @covers ::buildByExtension
    */