diff --git a/Gemfile b/Gemfile
index d821cdf6..8283a437 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,12 +5,18 @@ gemspec
 group :development do
   gem "mocha", "~> 0.13.2"
   gem "rake"
-  gem "rubocop"
   gem "shoulda-context"
   gem "test-unit"
   gem "timecop"
   gem "webmock"
 
+  # Rubocop changes pretty quickly: new cops get added and old cops change
+  # names or go into new namespaces. This is a library and we don't have
+  # `Gemfile.lock` checked in, so to prevent good builds from suddenly going
+  # bad, pin to a specific version number here. Try to keep this relatively
+  # up-to-date, but it's not the end of the world if it's not.
+  gem "rubocop", "0.50.0"
+
   # Rack 2.0+ requires Ruby >= 2.2.2 which is problematic for the test suite on
   # older Ruby versions. Check Ruby the version here and put a maximum
   # constraint on Rack if necessary.