undefined method unpack for nil:NilClass when rendering JSON - json

Question

I'm working on a Rails 5 API application and Im having trouble when trying to render json. Take that simple controller:
module Api
module V1
class UsersController < ApplicationController
def sign_up
#user = User.new user_params
if #user.save
render json: {status: :ok, user: #user.to_json }
else
render json: { status: :error, errors: #user.errors }, status: :unprocessable_entity
end
end
end
end
end
When I test this request with a simple spec:
RSpec.describe Api::V1::UsersController, type: :controller do
context 'As an unregistered user' do
context 'signing up with valid params' do
let(:valid_params) do
{ name: 'Test User', email: 'test#test.com', password: '123456789', password_confirmation: '123456789' }
end
it 'returns a 200 status' do
post :sign_up, params: { user: valid_params }
expect(response.status).to eq 200
end
end
end
end
I get an error thrown at me:
Failures:
1) Api::V1::UsersController As an unregistered user signing up with valid status returns a 200 status
Failure/Error: render json: {status: :ok, user: #user.to_json }
NoMethodError:
undefined method `unpack' for nil:NilClass
# ./app/controllers/api/v1/users_controller.rb:10:in `sign_up'
# ./spec/controllers/api/v1/users_controller_spec.rb:15:in `block (4 levels) in <top (required)>'
I'm not using any gems that alter rendering yet so I'm really lost of what might be the problem. Thanks in advance.