1 files changed, 1 insertions, 1 deletions
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
index 97c6e3e886..3d727bd0e6 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb
@@ -237,7 +237,7 @@ module ActiveRecord
 
       def checkin(conn)
         @connection_mutex.synchronize do
-          @checked_out -= conn
+          @checked_out.delete conn
           @queue.signal
         end
       end