diff --git a/core/modules/block/custom_block/custom_block.routing.yml b/core/modules/block/custom_block/custom_block.routing.yml
index fdffa13..2b3676c 100644
--- a/core/modules/block/custom_block/custom_block.routing.yml
+++ b/core/modules/block/custom_block/custom_block.routing.yml
@@ -14,6 +14,9 @@ custom_block_add_page:
     _permission: 'administer blocks'
 custom_block_add_form:
   pattern: block/add/{custom_block_type}
+  options:
+    converters:
+      custom_block_type: 'custom_block_type'
   defaults:
     _content: 'Drupal\custom_block\Controller\CustomBlockController::addForm'
   requirements: