diff --git a/jetty-util/src/main/java/org/eclipse/jetty/util/thread/ReservedThreadExecutor.java b/jetty-util/src/main/java/org/eclipse/jetty/util/thread/ReservedThreadExecutor.java
index 0d62fac9954..5010a631854 100644
--- a/jetty-util/src/main/java/org/eclipse/jetty/util/thread/ReservedThreadExecutor.java
+++ b/jetty-util/src/main/java/org/eclipse/jetty/util/thread/ReservedThreadExecutor.java
@@ -57,18 +57,18 @@ public class ReservedThreadExecutor extends AbstractLifeCycle implements Executo
     public ReservedThreadExecutor(Executor executor,int capacity)
     {
         _executor = executor;
-        
+
         if (capacity < 0)
         {
-            if (executor instanceof ThreadPool)
+            int cpus = Runtime.getRuntime().availableProcessors();
+            if (executor instanceof ThreadPool.SizedThreadPool)
             {
-                int threads = ((ThreadPool)executor).getThreads();
-                int cpus = Runtime.getRuntime().availableProcessors();
-                capacity = Math.max(1,Math.min(cpus,threads/8));
+                int threads = ((ThreadPool.SizedThreadPool)executor).getMaxThreads();
+                capacity = Math.max(1, Math.min(cpus, threads / 8));
             }
             else
             {
-                capacity = Runtime.getRuntime().availableProcessors();
+                capacity = cpus;
             }
         }