diff --git a/appengine/flexible/ruby31-and-earlier/static_files/sinatra/Gemfile.lock b/appengine/flexible/ruby31-and-earlier/static_files/sinatra/Gemfile.lock
index f4ae5a8e0..26eb8a797 100644
--- a/appengine/flexible/ruby31-and-earlier/static_files/sinatra/Gemfile.lock
+++ b/appengine/flexible/ruby31-and-earlier/static_files/sinatra/Gemfile.lock
@@ -11,7 +11,7 @@ GEM
       rack (>= 1.6.0)
       rack-test (>= 0.6.3)
       xpath (~> 3.1)
-    concurrent-ruby (1.2.2)
+    concurrent-ruby (1.3.7)
     cuprite (0.14.3)
       capybara (~> 3.0)
       ferrum (~> 0.13.0)