diff --git a/collects/mzlib/async-channel.ss b/collects/mzlib/async-channel.ss
index d51cb97..12f5c7a 100644
--- a/collects/mzlib/async-channel.ss
+++ b/collects/mzlib/async-channel.ss
@@ -4,7 +4,7 @@
 	   (lib "contract.ss"))
 
   (define-values (struct:ac make-ac async-channel? ac-ref ac-set!)
-    (make-struct-type 'async-channel #f 3 0 #f 
+    (make-struct-type 'async-channel #f 4 0 #f 
 		      (list (cons prop:waitable 
 				  ;; This is the guard that is called when
 				  ;;  we use an async-channel as a waitable
@@ -13,16 +13,18 @@
 				    ;; Make sure queue manager is running:
 				    (thread-resume (ac-thread ac) (current-thread))
 				    (ac-dequeue-ch ac))))
-		      (current-inspector) #f '(0)))
+		      (current-inspector) #f))
 
   (define ac-enqueue-ch (make-struct-field-accessor ac-ref 0))
   (define ac-dequeue-ch (make-struct-field-accessor ac-ref 1))
-  (define ac-thread (make-struct-field-accessor ac-ref 2))
+  (define ac-empty-ch (make-struct-field-accessor ac-ref 2))
+  (define ac-thread (make-struct-field-accessor ac-ref 3))
     
   (define make-async-channel
     (opt-lambda ([limit #f])
       (let* ([enqueue-ch (make-channel)]
 	     [dequeue-ch (make-channel)]
+	     [empty-ch (make-channel)]
 	     [queue-first (cons #f null)]
 	     [queue-last queue-first]
 	     [manager-thread
@@ -41,31 +43,42 @@
 			    (make-wrapped-waitable
 			     enqueue-ch
 			     (lambda (v)
-			       (let ([p (cons v null)])
+			       (let ([p (cons #f null)])
+				 (set-car! queue-last v)
 				 (set-cdr! queue-last p)
 				 (set! queue-last p)
-				 (loop)))))])
+				 (loop)))))]
+			 [mk-empty
+			  (lambda ()
+			    (make-wrapped-waitable
+			     (make-channel-put-waitable empty-ch #f)
+			     (lambda (ignored)
+			       (loop))))])
 		     (cond
 		      [(= 1 (length queue-first))
-		       (object-wait-multiple #f (mk-enqueue))]
+		       (object-wait-multiple #f (mk-enqueue) (mk-empty))]
 		      [(or (not limit) ((sub1 (length queue-first)) . < . limit))
 		       (object-wait-multiple #f (mk-enqueue) (mk-dequeue))]
 		      [else
 		       (object-wait-multiple #f (mk-dequeue))])))))])
-	(make-ac enqueue-ch dequeue-ch manager-thread))))
+	(make-ac enqueue-ch dequeue-ch empty-ch manager-thread))))
 
   (define (async-channel-get ac)
     (object-wait-multiple #f ac))
 
   (define (async-channel-try-get ac)
-    (object-wait-multiple 0 ac))
+    (object-wait-multiple #f ac (ac-empty-ch ac)))
 
   (define (make-async-channel-put-waitable ac v)
-    (make-guard-waitable
-     (lambda ()
-       ;; Make sure queue manager is running:
-       (thread-resume (ac-thread ac) (current-thread))
-       (make-channel-put-waitable (ac-enqueue-ch ac) v))))
+    (letrec ([p
+	      (make-wrapped-waitable
+	       (make-guard-waitable
+		(lambda ()
+		  ;; Make sure queue manager is running:
+		  (thread-resume (ac-thread ac) (current-thread))
+		  (make-channel-put-waitable (ac-enqueue-ch ac) v)))
+	       (lambda (ignored) p))])
+      p))
   
   (define (async-channel-put ac v)
     (object-wait-multiple #f (make-async-channel-put-waitable ac v))