diff --git a/lib/active_admin/namespace.rb b/lib/active_admin/namespace.rb
index 953b5b973d41..bb3f75a2a597 100644
--- a/lib/active_admin/namespace.rb
+++ b/lib/active_admin/namespace.rb
@@ -255,9 +255,15 @@ def build_page(name, options)
       resources.add Page.new(self, name, options)
     end
 
-    # TODO: replace `eval` with `Class.new`
     def register_page_controller(config)
-      eval "class ::#{config.controller_name} < ActiveAdmin::PageController; end"
+      if Object.const_defined?("::#{config.controller_name}")
+        unless config.controller.ancestors.include?(ActiveAdmin::PageController)
+          raise "#{config.controller_name} not a PageController"
+        end
+      else
+        eval "class ::#{config.controller_name} < ActiveAdmin::PageController; end"
+        config.controller_generated = true
+      end
       config.controller.active_admin_config = config
     end
 
@@ -265,7 +271,7 @@ def unload_resources!
       resources.each do |resource|
         parent = (module_name || 'Object').constantize
         name   = resource.controller_name.split('::').last
-        parent.send(:remove_const, name) if parent.const_defined?(name, false)
+        parent.send(:remove_const, name) if parent.const_defined?(name, false) && resource.controller_generated
 
         # Remove circular references
         resource.controller.active_admin_config = nil
@@ -285,7 +291,14 @@ def register_module
 
     # TODO: replace `eval` with `Class.new`
     def register_resource_controller(config)
-      eval "class ::#{config.controller_name} < ActiveAdmin::ResourceController; end"
+      if Object.const_defined?("::#{config.controller_name}")
+        unless config.controller.ancestors.include?(ActiveAdmin::ResourceController)
+          raise "#{config.controller_name} not a ResourceController"
+        end
+      else
+        eval "class ::#{config.controller_name} < ActiveAdmin::ResourceController; end"
+        config.controller_generated = true
+      end
       config.controller.active_admin_config = config
     end
 
diff --git a/lib/active_admin/page.rb b/lib/active_admin/page.rb
index 362525c4c7e2..a6824566ad8d 100644
--- a/lib/active_admin/page.rb
+++ b/lib/active_admin/page.rb
@@ -71,6 +71,8 @@ def controller_name
       [namespace.module_name, camelized_resource_name + "Controller"].compact.join('::')
     end
 
+    attr_accessor :controller_generated
+
     # Override from `ActiveAdmin::Resource::Controllers`
     def route_uncountable?
       false
diff --git a/lib/active_admin/resource/controllers.rb b/lib/active_admin/resource/controllers.rb
index 512f34d25e4b..e80a57452f5a 100644
--- a/lib/active_admin/resource/controllers.rb
+++ b/lib/active_admin/resource/controllers.rb
@@ -9,6 +9,8 @@ def controller_name
         [namespace.module_name, resource_name.plural.camelize + "Controller"].compact.join('::')
       end
 
+      attr_accessor :controller_generated
+
       # Returns the controller for this config
       def controller
         @controller ||= controller_name.constantize