rack-timeout: undefined method `state=' for nil:NilClass since 0.3.1

Yesterday we’ve upgraded rack-timeout to 0.3.1 from 0.2.4 and we started seeing this error on production (we’re using heroku) :

    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/core.rb:168 :in `_set_state!`

    166     def self._set_state!(env, state)
    167       raise "Invalid state: #{state.inspect}" unless VALID_STATES.include? state
    168       env[ENV_INFO_KEY].state = state
    169       notify_state_change_observers(env)
    170     end

    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/core.rb:115 :in `block in call`
    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/core.rb:117 :in `call`
    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/core.rb:117 :in `block in call`
    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/support/scheduler.rb:39 :in `call`
    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/support/scheduler.rb:39 :in `run!`
    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/support/scheduler.rb:52 :in `run!`
    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/support/scheduler.rb:99 :in `block (2 levels) in run_loop!`
    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/support/scheduler.rb:99 :in `each`
    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/support/scheduler.rb:99 :in `block in run_loop!`
    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/support/scheduler.rb:80 :in `loop`
    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/support/scheduler.rb:80 :in `run_loop!`
    [GEM_ROOT]/gems/rack-timeout-0.3.1/lib/rack/timeout/support/scheduler.rb:72 :in `block (2 levels) in runner`

Rack middlewares on production:

use Honeybadger::Rack::UserInformer
use Honeybadger::Rack::UserFeedback
use Honeybadger::Rack::ErrorNotifier
use HireFire::Middleware
use Rack::Cors
use Rack::UTF8Sanitizer
use Rack::Sendfile
use ActionDispatch::Static
use #<ActiveSupport::Cache::Strategy::LocalCache::Middleware:0x007f7d7d3117b0>
use Rack::Timeout
use Rack::Runtime
use Rack::MethodOverride
use ActionDispatch::RequestId
use Rails::Rack::Logger
use ActionDispatch::ShowExceptions
use ActionDispatch::DebugExceptions
use ActionDispatch::RemoteIp
use ActionDispatch::Callbacks
use ActionDispatch::Cookies
use ActionDispatch::Session::CookieStore
use ActionDispatch::Flash
use ActionDispatch::ParamsParser
use Rack::Head
use Rack::ConditionalGet
use Rack::ETag
use Warden::Manager
use JQuery::FileUpload::Rails::Middleware

Tried reproducing locally but I can’t seem to find the right conditions to do so.