diff --git a/actionpack/lib/action_dispatch/routing/mapper.rb b/actionpack/lib/action_dispatch/routing/mapper.rb
index 77740b0f53..2005382b8e 100644
--- a/actionpack/lib/action_dispatch/routing/mapper.rb
+++ b/actionpack/lib/action_dispatch/routing/mapper.rb
@@ -418,7 +418,9 @@ module ActionDispatch
           end
 
           def merge_blocks_scope(parent, child)
-            (parent || []) + [child]
+            merged = parent ? parent.dup : []
+            merged << child if child
+            merged
           end
 
           def merge_options_scope(parent, child)