diff --git a/book/Gemfile.lock b/book/Gemfile.lock
index 7bcd1e6..f6af4fd 100644
--- a/book/Gemfile.lock
+++ b/book/Gemfile.lock
@@ -156,6 +156,7 @@ GEM
     minitest (5.11.3)
     multi_json (1.13.1)
     multipart-post (2.0.0)
+    nio4r (2.5.9)
     nokogiri (1.8.1)
       mini_portile2 (~> 2.3.0)
     padrino-helpers (0.13.3.4)
@@ -166,7 +167,8 @@ GEM
       activesupport (>= 3.1)
     parallel (1.12.1)
     public_suffix (3.0.1)
-    puma (3.11.2)
+    puma (6.3.1)
+      nio4r (~> 2.0)
     rack (1.6.8)
     rack-livereload (0.3.16)
       rack