why am i getting an undefined method 'reviews' for nil:Nilclass when coding in ruby on rails?

I am getting this error when I click my link for "add review" in my book review application:

undefined method `reviews' for nil:NilClass

Trace of template inclusion: app/views/reviews/new.html.erb

app/views/reviews/_form.html.erb:1:in _app_views_reviews__form_html_erb___610767900_57241452' app/views/reviews/new.html.erb:5:in_app_views_reviews_new_html_erb__394574709_54781368'

Request

Parameters:

{"book_id"=>"4"}

How do I fix this?