Backtrace: ``` NoMethodError: undefined method `pop' for nil:NilClass /home/braulio/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/database_cleaner-1.5.3/lib/database_cleaner/sequel/transaction.rb:31:in `clean' /home/braulio/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/database_cleaner-1.5.3/lib/database_cleaner/base.rb:46:in `clean_with' /home/braulio/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/database_cleaner-1.5.3/lib/database_cleaner/configuration.rb:91:in `block in clean_with' /home/braulio/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/database_cleaner-1.5.3/lib/database_cleaner/configuration.rb:91:in `each' /home/braulio/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/database_cleaner-1.5.3/lib/database_cleaner/configuration.rb:91:in `clean_with' /home/braulio/Projects/olery/scheduler/spec/support/database_cleaner.rb:7:in `block (2 levels) in <top (required)>' ``` Multiple threads being used