diff -r ede0a342ed7e Lib/concurrent/futures/_base.py
--- a/Lib/concurrent/futures/_base.py	Thu Jan 23 16:49:22 2014 +0200
+++ b/Lib/concurrent/futures/_base.py	Fri Jan 24 08:51:59 2014 -0500
@@ -190,11 +190,12 @@
     if timeout is not None:
         end_time = timeout + time.time()
 
+    fs = set(fs)
     with _AcquireFutures(fs):
         finished = set(
                 f for f in fs
                 if f._state in [CANCELLED_AND_NOTIFIED, FINISHED])
-        pending = set(fs) - finished
+        pending = fs - finished
         waiter = _create_and_install_waiters(fs, _AS_COMPLETED)
 
     try: