diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
index 1d3953f..fac7abb 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscoveryParser.php
@@ -95,9 +95,7 @@ public function buildByExtension($extension) {
       elseif ($this->themeHandler->themeExists($extension)) {
         $extension_type = 'theme';
       }
-      else {
-        assert(FALSE, sprintf('The extension "%s" is not available.', $extension));
-      }
+      assert(isset($extension_type), sprintf('The extension "%s" is not available.', $extension));
       $path = $this->drupalGetPath($extension_type, $extension);
     }