diff --git a/lib/puma/thread_pool.rb b/lib/puma/thread_pool.rb
index 0746be6c..bbbad700 100644
--- a/lib/puma/thread_pool.rb
+++ b/lib/puma/thread_pool.rb
@@ -114,7 +114,7 @@ module Puma
 
         @todo << work
 
-        if @waiting == 0 and @spawned < @max
+        if @waiting < @todo.size and @spawned < @max
           spawn_thread
         end