async_exec can return nil

gistfile1.rb

require 'pg'

c = PG::Connection.open(dbname: 'database')
[
  Thread.new { Thread.current[:r] = c.async_exec("SELECT * FROM users") },
  Thread.new { Thread.current[:r] = c.async_exec("SELECT * FROM users") }
].each { |t| t.join; p t[:r] }