diff --git a/i18n.gemspec b/i18n.gemspec
index 738b8fd7..c52b2e8b 100644
--- a/i18n.gemspec
+++ b/i18n.gemspec
@@ -28,4 +28,20 @@ Gem::Specification.new do |s|
   s.required_ruby_version = '>= 2.0.0'
 
   s.add_dependency 'concurrent-ruby', '~> 1.0'
+
+  s.post_install_message = <<-END
+
+HEADS UP! i18n 1.1 changed fallbacks to exclude default locale.
+But that may break your application.
+
+Please check your Rails app for 'config.i18n.fallbacks = true'.
+If you're using I18n 1.1.x and Rails (< 6.0), this should be
+'config.i18n.fallbacks = [I18n.default_locale]'.
+If not, fallbacks will be broken in your app by I18n 1.1.x.
+
+For more info see:
+https://github.com/svenfuchs/i18n/releases/tag/v1.1.0
+
+END
+
 end