ActiveRecord::Base#exists? returns true when called with nil

Given the following models,

class Department < ActiveRecord::Base
  
  has_many :employees
  
end

class Employee < ActiveRecord::Base

  belongs_to :department

end

when determining if an employee belongs to a department using exists?, the following call will return true,

  dept = Department.first

  dept.employees.exists?(nil)

  ...

  => true

And according to the documentation,

Returns true if a record exists in the table that matches the id or conditions given, or false otherwise.

I'd assume in the example above, false should be returned if nil is passed.

tested this against Rails v3.0.3 and ruby 1.9.2.