Undefined method `write' for class `Jekyll::Post'

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I’m upgrading from Jekyll 2.2.0 to 3.6.0. Everything upgrades until I rake generate, when I get the following error:

## Generating Site with Jekyll
Configuration file: /var/www/_config.yml
jekyll 3.6.0 | Error:  undefined method `write' for class `Jekyll::Post'

Using ruby 2.4 with the following gems:

source 'http://rubygems.org'

gem 'jekyll'
gem 'jekyll-watch'
gem 'nokogiri'
gem 'nokogiri-pretty'
gem 'jekyll-paginate'
gem 'jekyll-coffeescript'
gem 'jekyll-gist'
gem 'pygments.rb'
gem 'redcarpet'
gem 'toml'
gem 'classifier-reborn'

What am I missing?