diff --git a/app/controllers/configurations_controller.rb b/app/controllers/configurations_controller.rb
--- a/app/controllers/configurations_controller.rb
+++ b/app/controllers/configurations_controller.rb
@@ -16,7 +16,7 @@
 
   def update
     @config = GraderConfiguration.find(params[:id])
-    User.clear_last_login if @config.key = 'multiple_ip_login' and @config.value == 'true' and params[:grader_configuration][:value] == 'false'
+    User.clear_last_login if @config.key == GraderConfiguration::MULTIPLE_IP_LOGIN_KEY and @config.value == 'true' and params[:grader_configuration][:value] == 'false'
     respond_to do |format|
       if @config.update_attributes(params[:grader_configuration])
         format.json { head :ok }