diff --git a/src/ExternalLibrary/LibraryManager.php b/src/ExternalLibrary/LibraryManager.php
index 45359e9..17a6223 100644
--- a/src/ExternalLibrary/LibraryManager.php
+++ b/src/ExternalLibrary/LibraryManager.php
@@ -61,11 +61,10 @@ class LibraryManager implements LibraryManagerInterface {
   public function getRequiredLibraryIds() {
     $library_ids = [];
     foreach (['module', 'theme'] as $type) {
-      // @todo Consider if this can be unit-testable by finding alterntaive to
-      // system_get_info() static call.
       foreach (system_get_info($type) as $info) {
-        assert('!empty($info)');
-        $library_ids += isset($info['library_dependencies']) ? $info['library_dependencies'] : [];
+        if (isset($info['library_dependencies'])) {
+          $library_ids = array_merge($library_ids, $info['library_dependencies']);
+        }
       }
     }
     return array_unique($library_ids);
diff --git a/tests/src/Kernel/ExternalLibrary/Asset/AssetLibraryTestBase.php b/tests/src/Kernel/ExternalLibrary/Asset/AssetLibraryTestBase.php
index 5fe9601..4f835b7 100644
--- a/tests/src/Kernel/ExternalLibrary/Asset/AssetLibraryTestBase.php
+++ b/tests/src/Kernel/ExternalLibrary/Asset/AssetLibraryTestBase.php
@@ -12,8 +12,9 @@ abstract class AssetLibraryTestBase extends LibraryTypeKernelTestBase {
   /**
    * {@inheritdoc}
    *
-   * \Drupal\libraries\ExternalLibrary requires system_get_info() which is in
-   * system.module.
+   * LibraryManager requires system_get_info() which is in system.module.
+   *
+   * @see \Drupal\libraries\ExternalLibrary\LibraryManager::getRequiredLibraryIds()
    */
   public static $modules = ['system'];