diff --git a/lib/puma/thread_pool.rb b/lib/puma/thread_pool.rb
index 8e25162c..abada439 100644
--- a/lib/puma/thread_pool.rb
+++ b/lib/puma/thread_pool.rb
@@ -119,6 +119,7 @@ module Puma
                 @trim_requested -= 1
                 @spawned -= 1
                 @workers.delete th
+                not_full.signal
                 Thread.exit
               end