diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_specification.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_specification.rb
index b3ba99a013..a65d6df6a1 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_specification.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_specification.rb
@@ -98,7 +98,7 @@ module ActiveRecord
           stale = Set.new(cache.keys)
 
           Thread.list.each do |thread|
-            stale -= thread.object_id if t.alive?
+            stale.delete(thread.object_id) if thread.alive?
           end
 
           stale.each do |thread_id|