diff --git a/railties/test/isolation/abstract_unit.rb b/railties/test/isolation/abstract_unit.rb
index bf0fdea89a4c41bf696d29266533be2f5213e908..fe173e715981577e75d7b2217805599f1ceef8f0 100644
--- a/railties/test/isolation/abstract_unit.rb
+++ b/railties/test/isolation/abstract_unit.rb
@@ -93,7 +93,8 @@ module Generation
     # Build an application by invoking the generator and going through the whole stack.
     def build_app(options = {})
       @prev_rails_env = ENV['RAILS_ENV']
-      ENV['RAILS_ENV'] = 'development'
+      ENV['RAILS_ENV']             = 'development'
+      ENV['RAILS_SECRET_KEY_BASE'] ||= SecureRandom.hex(16)
 
       FileUtils.rm_rf(app_path)
       FileUtils.cp_r(app_template_path, app_path)