diff --git a/docker/exercises/sample_app/Gemfile b/docker/exercises/sample_app/Gemfile
index 1d0a7a4..d4e2f61 100644
--- a/docker/exercises/sample_app/Gemfile
+++ b/docker/exercises/sample_app/Gemfile
@@ -7,7 +7,7 @@ ruby '2.5.1'
 gem 'rails', '~> 5.2.1'
 gem 'jquery-rails', '~> 4.3', '>= 4.3.3'
 gem 'pg', '>= 0.18', '< 2.0'
-gem 'puma', '~> 3.11'
+gem 'puma', '~> 4.3'
 gem 'sass-rails', '~> 5.0'
 gem 'uglifier', '>= 1.3.0'
 gem 'coffee-rails', '~> 4.2'
diff --git a/docker/exercises/sample_app/Gemfile.lock b/docker/exercises/sample_app/Gemfile.lock
index 02facfa..439ee79 100644
--- a/docker/exercises/sample_app/Gemfile.lock
+++ b/docker/exercises/sample_app/Gemfile.lock
@@ -107,7 +107,9 @@ GEM
       mimemagic (~> 0.3.2)
     memcachier (0.0.2)
     method_source (0.9.0)
-    mimemagic (0.3.2)
+    mimemagic (0.3.10)
+      nokogiri (~> 1)
+      rake
     mini_mime (1.0.1)
     mini_portile2 (2.3.0)
     minitest (5.11.3)
@@ -123,7 +125,8 @@ GEM
     pry-rails (0.3.6)
       pry (>= 0.10.4)
     public_suffix (3.0.3)
-    puma (3.12.0)
+    puma (4.3.12)
+      nio4r (~> 2.0)
     rack (2.0.5)
     rack-protection (2.0.3)
       rack
@@ -228,7 +231,7 @@ DEPENDENCIES
   nokogiri (~> 1.8, >= 1.8.4)
   pg (>= 0.18, < 2.0)
   pry-rails (~> 0.3.6)
-  puma (~> 3.11)
+  puma (~> 4.3)
   rails (~> 5.2.1)
   redis (~> 4.0, >= 4.0.2)
   sass-rails (~> 5.0)