abilities.ex

defmodule Blog.Abilities do                                                                                                                                                                                                                
  defimpl Canada.Can, for: Blog.User do                                                                                                                                                                                                    
    def can?(%Blog.User{}, :index, Blog.User), do: true                                                                                                                                                                                    
    def can?(%Blog.User{}, _, _), do: true                                                                                                                                                                                                 
    def can?(nil, _, _), do: false                                                                                                                                                                                                         
  end
  
  defimpl Canada.Can, for: nil do                                                                                                                                                                                                          
    def can?(nil, _, _), do: false
  end
end                                                                                                                                                                                                                                       end

When defimpl Canada.Can, for: nil... is removed, the protocol resolved fine for normal users, but not for nil