diff --git a/Gemfile b/Gemfile
index 1b657e7d..ae5ff138 100644
--- a/Gemfile
+++ b/Gemfile
@@ -55,6 +55,7 @@ if RUBY_ENGINE == "ruby"
   gem 'haml', '>= 3.0'
   gem 'sass'
   gem 'reel-rack'
+  gem 'celluloid', '~> 0.16.0'
 end
 
 if RUBY_ENGINE == "rbx"
diff --git a/sinatra.gemspec b/sinatra.gemspec
index d75f95ce..1ee8ce02 100644
--- a/sinatra.gemspec
+++ b/sinatra.gemspec
@@ -15,7 +15,7 @@ Gem::Specification.new 'sinatra', Sinatra::VERSION do |s|
 
   s.required_ruby_version = '>= 2.2.0'
 
-  s.add_dependency 'rack', '= 2.0.0.alpha'
+  s.add_dependency 'rack', '= 2.0.0.rc1'
   s.add_dependency 'tilt', '~> 2.0'
   s.add_dependency 'rack-protection', '~> 1.5'
   s.add_dependency 'mustermann',  '~> 0.4'