diff --git a/core/modules/migrate_drupal/config/install/migrate.migration.d6_user.yml b/core/modules/migrate_drupal/config/install/migrate.migration.d6_user.yml
index af2bceb..33850c8 100644
--- a/core/modules/migrate_drupal/config/install/migrate.migration.d6_user.yml
+++ b/core/modules/migrate_drupal/config/install/migrate.migration.d6_user.yml
@@ -23,12 +23,15 @@ process:
   signature: signature
   signature_format:
     -
+# Drupal 6 stores 0 as the signature format when signatures are disabled.
+# Drupal 8 stores a NULL.
       plugin: static_map
       bypass: true
       source: signature_format
       map:
         0: NULL
     -
+# Do not try to migrate the NULL format.
       plugin: skip_process_on_empty
     -
       plugin: migration