diff --git a/akka-actor/src/main/scala/akka/dispatch/Mailbox.scala b/akka-actor/src/main/scala/akka/dispatch/Mailbox.scala
index 322b50b900..f5a704dfb2 100644
--- a/akka-actor/src/main/scala/akka/dispatch/Mailbox.scala
+++ b/akka-actor/src/main/scala/akka/dispatch/Mailbox.scala
@@ -46,9 +46,26 @@ private[akka] abstract class Mailbox(val actor: ActorCell, val messageQueue: Mes
 
   import Mailbox._
 
+  /**
+   * Try to enqueue the message to this queue, or throw an exception.
+   */
   def enqueue(receiver: ActorRef, msg: Envelope): Unit = messageQueue.enqueue(receiver, msg)
+  
+  /**
+   * Try to dequeue the next message from this queue, return null failing that.
+   */
   def dequeue(): Envelope = messageQueue.dequeue()
+
+  /**
+   * Indicates whether this queue is non-empty.
+   */
   def hasMessages: Boolean = messageQueue.hasMessages
+
+  /**
+   * Should return the current number of messages held in this queue; may
+   * always return 0 if no other value is available efficiently. Do not use
+   * this for testing for presence of messages, use `hasMessages` instead.
+   */
   def numberOfMessages: Int = messageQueue.numberOfMessages
 
   @volatile
diff --git a/akka-testkit/src/main/scala/akka/testkit/CallingThreadDispatcher.scala b/akka-testkit/src/main/scala/akka/testkit/CallingThreadDispatcher.scala
index 84ea7aa197..2fe664d7b6 100644
--- a/akka-testkit/src/main/scala/akka/testkit/CallingThreadDispatcher.scala
+++ b/akka-testkit/src/main/scala/akka/testkit/CallingThreadDispatcher.scala
@@ -294,7 +294,7 @@ class CallingThreadMailbox(_receiver: ActorCell, val mailboxType: MailboxType) e
   override def enqueue(receiver: ActorRef, msg: Envelope): Unit = throw new UnsupportedOperationException("CallingThreadMailbox cannot enqueue normally")
   override def dequeue(): Envelope = throw new UnsupportedOperationException("CallingThreadMailbox cannot dequeue normally")
   override def hasMessages: Boolean = q.get.q.hasMessages
-  override def numberOfMessages: Int = -1
+  override def numberOfMessages: Int = 0
 
   def queue = q.get