Plugin Step Assign Message - No Assignee input parameter

I have created an Account plugin which has a step registered for the Assign message (pre-operation). The Plugin fires when I re-assign an account, but it appears there is no Assignee inputparameter.

CRM 2013 online

if(context.InputParameters.Contains("Assignee")) -- This returns false.

I am able to grab my entity from the context.

Everything I've read says I should be able to get an entity reference from the Assignee but no luck.

Any ideas.