Error When Declaring roleManager

I currently had this for defining the roleManager:
var roleManager = new RoleManager<ApplicationRole>(new RoleStore<ApplicationRole>(context));

Your code says to use this:
var roleManager = HttpContext.Current.GetOwinContext().Get<ApplicationRoleManager>();

I'm getting an error for the ApplicationRoleManager saying this:
There is no argument given that corresponds to the required formal parameter 'key' of 'IOwinContect.Get(string)'

All of my other code is the same. Do you know what the issue could be? I have defined the ApplicationRoleManager exactly like your code.