delayed_job - Undefined method error even with Mongrel

Any delayed_job gurus out there? This is a known issue on the wiki as described at the bottom here:

However, even when running Mongrel (on Rails 3.0.3, ruby 1.9.2, Win7) after replacing Webrick) I am still getting this issue. My delayed_job handlers read:

--- !ruby/struct:Delayed::PerformableMailer object: !ruby/object:Class UserMailer

when it should read:

--- !ruby/struct:Delayed::PerformableMailer object: !ruby/class UserMailer

Is there a solution to this problem other than running mongrel? Because I am running mongrel and still have the same problem. Thanks in advance!