diff --git a/src/wormhole/blocking/transcribe.py b/src/wormhole/blocking/transcribe.py
index 82be96c..23ad5b0 100644
--- a/src/wormhole/blocking/transcribe.py
+++ b/src/wormhole/blocking/transcribe.py
@@ -10,6 +10,7 @@ from .. import __version__
 from .. import codes
 from ..errors import ServerError, Timeout, WrongPasswordError, UsageError
 from ..util.hkdf import HKDF
+from ..channel_monitor import monitor
 
 SECOND = 1
 MINUTE = 60*SECOND
@@ -141,7 +142,6 @@ class Wormhole:
         self.verifier = None
         self._sent_data = False
         self._got_data = False
-        self._closed = False
 
     def handle_welcome(self, welcome):
         if ("motd" in welcome and
@@ -194,6 +194,7 @@ class Wormhole:
         self.code = code
         channelid = int(mo.group(1))
         self.channel = self._channel_manager.connect(channelid)
+        monitor.add(self.channel)
 
     def _start(self):
         # allocate the rest now too, so it can be serialized
@@ -263,9 +264,5 @@ class Wormhole:
             raise WrongPasswordError
 
     def close(self):
+        monitor.close(self.channel)
         self.channel.deallocate()
-        self._closed = True
-
-    def __del__(self):
-        if not self._closed:
-            print("Error: a Wormhole instance was not closed", file=sys.stderr)
diff --git a/src/wormhole/channel_monitor.py b/src/wormhole/channel_monitor.py
new file mode 100644
index 0000000..964b2a5
--- /dev/null
+++ b/src/wormhole/channel_monitor.py
@@ -0,0 +1,16 @@
+from __future__ import print_function
+import sys
+from weakref import ref
+
+class ChannelMonitor:
+    def __init__(self):
+        self._open_channels = set()
+    def add(self, w):
+        wr = ref(w, self._lost)
+        self._open_channels.add(wr)
+    def _lost(self, wr):
+        print("Error: a Wormhole instance was not closed", file=sys.stderr)
+    def close(self, w):
+        self._open_channels.discard(ref(w))
+
+monitor = ChannelMonitor() # singleton
diff --git a/src/wormhole/twisted/transcribe.py b/src/wormhole/twisted/transcribe.py
index 742999d..89ea7ca 100644
--- a/src/wormhole/twisted/transcribe.py
+++ b/src/wormhole/twisted/transcribe.py
@@ -15,6 +15,7 @@ from .. import __version__
 from .. import codes
 from ..errors import ServerError, WrongPasswordError, UsageError
 from ..util.hkdf import HKDF
+from ..channel_monitor import monitor
 
 @implementer(IBodyProducer)
 class DataProducer:
@@ -152,7 +153,6 @@ class Wormhole:
         self._started_get_code = False
         self._sent_data = False
         self._got_data = False
-        self._closed = False
 
     def _set_side(self, side):
         self._side = side
@@ -209,6 +209,7 @@ class Wormhole:
         self.code = code
         channelid = int(mo.group(1))
         self.channel = self._channel_manager.connect(channelid)
+        monitor.add(self.channel)
 
     def _start(self):
         # allocate the rest now too, so it can be serialized
@@ -325,13 +326,6 @@ class Wormhole:
         return d
 
     def close(self, res=None):
+        monitor.close(self.channel)
         d = self.channel.deallocate()
-        def _closed(_):
-            self._closed = True
-            return res
-        d.addCallback(_closed)
         return d
-
-    def __del__(self):
-        if not self._closed:
-            print("Error: a Wormhole instance was not closed", file=sys.stderr)