Rails: Querying has_one through belongs_to adds PK null in query - ruby-on-rails

Question

I'm getting an error when trying to query a relation. These are my models:
class Course < ApplicationRecord
belongs_to :subject, inverse_of: :courses
# This is the important relation
has_one :department, through: :subject
end
class Subject < ApplicationRecord
belongs_to :department, inverse_of: :subjects
has_many :courses, dependent: :destroy, inverse_of: :subject
end
class Department < ApplicationRecord
has_many :subjects, dependent: :destroy
end
The thing is that if I do:
> Course.where(department: Department.first)
Department Load (0.3ms) SELECT `departments`.* FROM `departments` ORDER BY `departments`.`id` ASC LIMIT 1
Course Load (0.7ms) SELECT `courses`.* FROM `courses` WHERE `courses`.`id` IS NULL
=> []
As you can see, it includes a WHERE courses.id IS NULL. I'm able anyway to access this relation by doing:
> Course.joins(:subject).where(subjects: { department: Department.first })
Department Load (0.4ms) SELECT `departments`.* FROM `departments` ORDER BY `departments`.`id` ASC LIMIT 1
Course Load (0.5ms) SELECT `courses`.* FROM `courses` INNER JOIN `subjects` ON `subjects`.`id` = `courses`.`subject_id` WHERE `subjects`.`department_id` = 1
And I get many objects as result, but I'd like to know if I'm missing something to get this to work.
BTW, if I try to access the method directly, it works correctly:
> Course.first.department
Course Load (0.9ms) SELECT `courses`.* FROM `courses` ORDER BY `courses`.`id` ASC LIMIT 1
Department Load (0.6ms) SELECT `departments`.* FROM `departments` INNER JOIN `subjects` ON `departments`.`id` = `subjects`.`department_id` WHERE `subjects`.`id` = 1 LIMIT 1
=> #<Department:0x00007fc40dfab5f8
id: 1,
name: "Matemática",
slug: "matematica",
code: 1,
created_at: Sat, 15 Jun 2019 20:49:33 UTC +00:00,
updated_at: Sat, 15 Jun 2019 20:49:33 UTC +00:00>
Thank you!