diff --git a/core/modules/migrate_drupal/src/Tests/d6/MigrateFileTest.php b/core/modules/migrate_drupal/src/Tests/d6/MigrateFileTest.php
index 6e383bc..91560a4 100644
--- a/core/modules/migrate_drupal/src/Tests/d6/MigrateFileTest.php
+++ b/core/modules/migrate_drupal/src/Tests/d6/MigrateFileTest.php
@@ -21,6 +21,11 @@
  */
 class MigrateFileTest extends MigrateDrupal6TestBase implements MigrateDumpAlterInterface {
 
+  /**
+   * The filename of a file used to test temporary file migration.
+   *
+   * @var string
+   */
   protected static $tempFilename;
 
   /**