diff --git a/padrino-core/padrino-core.gemspec b/padrino-core/padrino-core.gemspec
index 010eae412..cd5ee5bfd 100755
--- a/padrino-core/padrino-core.gemspec
+++ b/padrino-core/padrino-core.gemspec
@@ -21,6 +21,7 @@ Gem::Specification.new do |s|
   s.rdoc_options  = ["--charset=UTF-8"]
 
   s.add_dependency("padrino-support", Padrino.version)
+  s.add_dependency("rackup", "~> 2.1")
   s.add_dependency("sinatra", "~> 4")
   s.add_dependency("thor", "~> 1.0")
 end