[dancer-users] Setting Session Engine

I am trying to get Deferred to work but keep getting: Error 500: core - Must specify session engine in settings prior to using 'session' keyword at /home/richard/perl5/lib/perl5/Dancer/Plugin/Deferred.pm line 26. Powered by Dancer2 <http://perldancer.org/> 0.161000 I have used the command: set session => 'YAML'; to set the session. I have also tried session: "YAML" in my config file. Is there something else I must do?