--- streams2.py	2015-06-11 16:49:14.851855051 +0200
+++ streams.py	2015-06-11 16:50:14.772299457 +0200
@@ -194,9 +194,9 @@
         if not self._paused:
             return
         waiter = self._drain_waiter
-        assert waiter is None or waiter.cancelled()
-        waiter = futures.Future(loop=self._loop)
-        self._drain_waiter = waiter
+        if waiter is None or waiter.cancelled():
+            waiter = futures.Future(loop=self._loop)
+            self._drain_waiter = waiter
         yield from waiter