diff --git a/actionpack/test/abstract_unit.rb b/actionpack/test/abstract_unit.rb
index 5ae54d097a4f695ed9b9a5c5f2f949921ae7042f..c71da7fa6caeb8ea513f52c4f006ad05d6bdc595 100644
--- a/actionpack/test/abstract_unit.rb
+++ b/actionpack/test/abstract_unit.rb
@@ -1,3 +1,7 @@
+if ENV['new_base']
+  puts *caller
+  raise 'new_base/abstract_unit already loaded'
+end
 $:.unshift(File.dirname(__FILE__) + '/../lib')
 $:.unshift(File.dirname(__FILE__) + '/../../activesupport/lib')
 $:.unshift(File.dirname(__FILE__) + '/fixtures/helpers')