diff --git a/Gemfile b/Gemfile
index bccab3d..2a57570 100644
--- a/Gemfile
+++ b/Gemfile
@@ -2,7 +2,7 @@ source 'https://rubygems.org'
 ruby '2.4.1'
 
 # HTTP server for Ruby/Rack recommended by Heroku
-gem 'puma', '~> 3.11'
+gem 'puma', '~> 4.2'
 
 # Middleman
 gem 'middleman', '~> 4.2', '>= 4.2.1'
diff --git a/Gemfile.lock b/Gemfile.lock
index fd288f2..e9d9bb6 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -82,6 +82,7 @@ GEM
       middleman-core (>= 3.3)
       rack-livereload (~> 0.3.15)
     minitest (5.10.3)
+    nio4r (2.5.2)
     padrino-helpers (0.13.3.4)
       i18n (~> 0.6, >= 0.6.7)
       padrino-support (= 0.13.3.4)
@@ -90,7 +91,8 @@ GEM
       activesupport (>= 3.1)
     parallel (1.12.0)
     public_suffix (3.0.0)
-    puma (3.11.2)
+    puma (4.2.0)
+      nio4r (~> 2.0)
     rack (2.0.3)
     rack-contrib (2.0.1)
       rack (~> 2.0)
@@ -121,7 +123,7 @@ DEPENDENCIES
   itcsscli (~> 0.1.15)
   middleman (~> 4.2, >= 4.2.1)
   middleman-livereload (~> 3.4, >= 3.4.6)
-  puma (~> 3.11)
+  puma (~> 4.2)
   rack-contrib (~> 2.0)
   slim (~> 3.0, >= 3.0.8)
   tzinfo-data