diff --git a/akka-testkit/src/main/scala/akka/testkit/CallingThreadDispatcher.scala b/akka-testkit/src/main/scala/akka/testkit/CallingThreadDispatcher.scala
index 6b8ea3b5df..5ecdcf3948 100644
--- a/akka-testkit/src/main/scala/akka/testkit/CallingThreadDispatcher.scala
+++ b/akka-testkit/src/main/scala/akka/testkit/CallingThreadDispatcher.scala
@@ -50,7 +50,11 @@ private[testkit] class CallingThreadDispatcherQueues extends Extension {
 
   // we have to forget about long-gone threads sometime
   private def gc {
-    queues = queues mapValues (_ filter (_.get ne null)) filter (!_._2.isEmpty)
+    queues = (Map.newBuilder[CallingThreadMailbox, Set[WeakReference[NestingQueue]]] /: queues) {
+      case (m, (k, v)) ⇒
+        val nv = v filter (_.get ne null)
+        if (nv.isEmpty) m else m += (k -> nv)
+    }.result
   }
 
   protected[akka] def registerQueue(mbox: CallingThreadMailbox, q: NestingQueue): Unit = synchronized {
@@ -67,6 +71,10 @@ private[testkit] class CallingThreadDispatcherQueues extends Extension {
     }
   }
 
+  protected[akka] def unregisterQueues(mbox: CallingThreadMailbox): Unit = synchronized {
+    queues -= mbox
+  }
+
   /*
    * This method must be called with "own" being this thread's queue for the
    * given mailbox. When this method returns, the queue will be entered
@@ -151,10 +159,21 @@ class CallingThreadDispatcher(
     }
   }
 
+  protected[akka] override def unregister(actor: ActorCell): Unit = {
+    val mbox = actor.mailbox match {
+      case m: CallingThreadMailbox ⇒ Some(m)
+      case _                       ⇒ None
+    }
+    super.unregister(actor)
+    mbox foreach CallingThreadDispatcherQueues(actor.system).unregisterQueues
+  }
+
   override def suspend(actor: ActorCell) {
     actor.mailbox match {
-      case m: CallingThreadMailbox ⇒ m.suspendSwitch.switchOn; m.suspend()
-      case m                       ⇒ m.systemEnqueue(actor.self, Suspend())
+      case m: CallingThreadMailbox ⇒
+        m.suspendSwitch.switchOn; m.suspend()
+      case m ⇒
+        m.systemEnqueue(actor.self, Suspend())
     }
   }
 
@@ -319,10 +338,11 @@ class CallingThreadMailbox(_receiver: akka.actor.Cell, val mailboxType: MailboxT
      * the gather operation, tough luck: no guaranteed delivery to deadLetters.
      */
     suspendSwitch.locked {
-      val q = queue
-      CallingThreadDispatcherQueues(actor.system).gatherFromAllOtherQueues(this, q)
+      val qq = queue
+      CallingThreadDispatcherQueues(actor.system).gatherFromAllOtherQueues(this, qq)
       super.cleanUp()
-      q.q.cleanUp(actor.self, actor.systemImpl.deadLetterQueue)
+      qq.q.cleanUp(actor.self, actor.systemImpl.deadLetterQueue)
+      q.remove()
     }
   }
 }