diff --git a/Chapter11/Exercise11.04/Gemfile b/Chapter11/Exercise11.04/Gemfile
index accdd86..7de3134 100644
--- a/Chapter11/Exercise11.04/Gemfile
+++ b/Chapter11/Exercise11.04/Gemfile
@@ -11,7 +11,7 @@ gem 'rails', '~> 5.2.3'
 # Use sqlite3 as the database for Active Record
 gem 'sqlite3'
 # Use Puma as the app server
-gem 'puma', '~> 3.7'
+gem 'puma', '~> 4.3'
 # Use SCSS for stylesheets
 gem 'sass-rails', '~> 5.0'
 # Use Uglifier as compressor for JavaScript assets
diff --git a/Chapter11/Exercise11.04/Gemfile.lock b/Chapter11/Exercise11.04/Gemfile.lock
index 6aee8a5..f0f4eb0 100644
--- a/Chapter11/Exercise11.04/Gemfile.lock
+++ b/Chapter11/Exercise11.04/Gemfile.lock
@@ -88,7 +88,9 @@ GEM
     marcel (0.3.3)
       mimemagic (~> 0.3.2)
     method_source (0.9.2)
-    mimemagic (0.3.3)
+    mimemagic (0.3.10)
+      nokogiri (~> 1)
+      rake
     mini_mime (1.0.2)
     mini_portile2 (2.4.0)
     minitest (5.11.3)
@@ -99,7 +101,8 @@ GEM
       coderay (~> 1.1.0)
       method_source (~> 0.9.0)
     public_suffix (4.0.1)
-    puma (3.12.1)
+    puma (4.3.12)
+      nio4r (~> 2.0)
     rack (2.0.7)
     rack-test (1.1.0)
       rack (>= 1.0, < 3)
@@ -190,7 +193,7 @@ DEPENDENCIES
   jbuilder (~> 2.5)
   listen (>= 3.0.5, < 3.2)
   pry (~> 0.12.2)
-  puma (~> 3.7)
+  puma (~> 4.3)
   rails (~> 5.2.3)
   sass-rails (~> 5.0)
   selenium-webdriver