diff --git a/Gemfile.lock b/Gemfile.lock
index 31ace42..13d5d9e 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -4,7 +4,7 @@ PATH
     smashing (1.3.6)
       coffee-script (~> 2.4)
       execjs (~> 2.7)
-      rack (~> 2.2)
+      rack (>= 2.2, < 4.0)
       rufus-scheduler (~> 3.6)
       sassc (~> 2.0)
       sinatra (~> 2.0)
@@ -44,7 +44,7 @@ GEM
     mustermann (2.0.2)
       ruby2_keywords (~> 0.0.1)
     raabro (1.4.0)
-    rack (2.2.3)
+    rack (2.2.4)
     rack-protection (2.2.3)
       rack
     rack-test (0.6.3)
diff --git a/smashing.gemspec b/smashing.gemspec
index 713f070..2eef006 100644
--- a/smashing.gemspec
+++ b/smashing.gemspec
@@ -44,7 +44,7 @@ Gem::Specification.new do |s|
     s.add_dependency('sprockets', '~> 4.0')
     s.add_dependency('sassc', '~> 2.0')
   end
-  s.add_dependency('rack', '~> 2.2')
+  s.add_dependency('rack', '>= 2.2', '< 4.0')
 
   s.add_development_dependency('rake', '~> 12.3.3')
   s.add_development_dependency('haml', '~> 5.0.1')