diff --git a/lib/perfectqueue/engine.rb b/lib/perfectqueue/engine.rb
index <HASH>..<HASH> 100644
--- a/lib/perfectqueue/engine.rb
+++ b/lib/perfectqueue/engine.rb
@@ -56,7 +56,7 @@ module PerfectQueue
           @processors << @processor_class.new(@runner, @processors.size+1, config)
         end
       elsif extra < 0
-        -extra.times do
+        (-extra).times do
           c = @processors.shift
           c.stop(immediate)
           c.join
- is weaker than .; needs parenthesis