diff --git a/tests/parent_test.py b/tests/parent_test.py
index 797845df..0bd8079c 100644
--- a/tests/parent_test.py
+++ b/tests/parent_test.py
@@ -31,8 +31,25 @@ def wait_for_child(pid, timeout=1.0):
 
 
 @mitogen.core.takes_econtext
-def call_func_in_sibling(ctx, econtext):
-    ctx.call(time.sleep, 99999)
+def call_func_in_sibling(ctx, econtext, sync_sender):
+    recv = ctx.call_async(time.sleep, 99999)
+    sync_sender.send(None)
+    recv.get().unpickle()
+
+
+def wait_for_empty_output_queue(sync_recv, context):
+    # wait for sender to submit their RPC. Since the RPC is sent first, the
+    # message sent to this sender cannot arrive until we've routed the RPC.
+    sync_recv.get()
+
+    router = context.router
+    broker = router.broker
+    while True:
+        # Now wait for the RPC to exit the output queue.
+        stream = router.stream_by_id(context.context_id)
+        if broker.defer_sync(lambda: stream.pending_bytes()) == 0:
+            return
+        time.sleep(0.1)
 
 
 class GetDefaultRemoteNameTest(testlib.TestCase):
@@ -353,12 +370,17 @@ class DisconnectTest(testlib.RouterMixin, testlib.TestCase):
         self.router.stream_by_id(c1.context_id).auth_id = mitogen.context_id
         c1.call(mitogen.parent.upgrade_router)
 
-        recv = c1.call_async(call_func_in_sibling, c2)
+        sync_recv = mitogen.core.Receiver(self.router)
+        recv = c1.call_async(call_func_in_sibling, c2,
+            sync_sender=sync_recv.to_sender())
+
+        wait_for_empty_output_queue(sync_recv, c2)
         c2.shutdown(wait=True)
+
         e = self.assertRaises(mitogen.core.CallError,
             lambda: recv.get().unpickle())
         s = 'mitogen.core.ChannelError: ' + self.router.respondent_disconnect_msg
-        self.assertTrue(e.args[0].startswith(s))
+        self.assertTrue(e.args[0].startswith(s), str(e))
 
     def test_far_sibling_disconnected(self):
         # God mode: child of child notices child of child of parent has
@@ -373,8 +395,13 @@ class DisconnectTest(testlib.RouterMixin, testlib.TestCase):
         self.router.stream_by_id(c1.context_id).auth_id = mitogen.context_id
         c11.call(mitogen.parent.upgrade_router)
 
-        recv = c11.call_async(call_func_in_sibling, c22)
+        sync_recv = mitogen.core.Receiver(self.router)
+        recv = c11.call_async(call_func_in_sibling, c22,
+            sync_sender=sync_recv.to_sender())
+
+        wait_for_empty_output_queue(sync_recv, c22)
         c22.shutdown(wait=True)
+
         e = self.assertRaises(mitogen.core.CallError,
             lambda: recv.get().unpickle())
         s = 'mitogen.core.ChannelError: ' + self.router.respondent_disconnect_msg