diff --git a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
index b0c3696..ccfecef 100644
--- a/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
+++ b/core/lib/Drupal/Core/Asset/LibraryDiscovery.php
@@ -45,6 +45,9 @@ public function getLibrariesByExtension($extension) {
    */
   public function getLibraryByName($extension, $name) {
     $extension = $this->getLibrariesByExtension($extension);
+    if (!isset($extension[$name])) {
+      throw new \Exception("Missing library $extension:$name");
+    }
     return isset($extension[$name]) ? $extension[$name] : FALSE;
   }