Rails 5.1 requires ActionController::TestRequest to have a controller class

Pass it in as an anonymous class

test/rails_app/config/boot.rb

@@ -3,7 +3,11 @@ unless defined?(DEVISE_ORM)
 end
 
 module Devise
-  # Detection for minor differences between Rails 4 and 5 in tests.
+  # Detection for minor differences between Rails 4 and 5, and 5.1 in tests.
+  def self.rails51?
+    Rails.version.start_with? '5.1'
+  end
+
   def self.rails5?
     Rails.version.start_with? '5'
   end