diff --git a/tool/sync_default_gems.rb b/tool/sync_default_gems.rb
index 1fed998af4..4f80db80b2 100644
--- a/tool/sync_default_gems.rb
+++ b/tool/sync_default_gems.rb
@@ -536,6 +536,9 @@ else
     when "-a"
       auto = true
       ARGV.shift
+    else
+      $stderr.puts "Unknown command line option: #{ARGV[0]}"
+      exit 1
     end
   end
   gem = ARGV.shift