diff --git a/src/java/com/samskivert/util/Invoker.java b/src/java/com/samskivert/util/Invoker.java
index 487e868c..f0ad601c 100644
--- a/src/java/com/samskivert/util/Invoker.java
+++ b/src/java/com/samskivert/util/Invoker.java
@@ -136,6 +136,9 @@ public class Invoker extends LoopingThread
      */
     public void postUnit (Unit unit)
     {
+        if (!isRunning()) {
+            throw new IllegalStateException("Invoker has been shutdown");
+        }
         // note the time
         unit.queueStamp = System.currentTimeMillis();
         // and append it to the queue