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	Thu Jan 23 14:12:33 2014 -0500
@@ -251,10 +251,11 @@
         completed. The second set, named 'not_done', contains uncompleted
         futures.
     """
+    fs = set(fs)
     with _AcquireFutures(fs):
         done = set(f for f in fs
                    if f._state in [CANCELLED_AND_NOTIFIED, FINISHED])
-        not_done = set(fs) - done
+        not_done = fs - done
 
         if (return_when == FIRST_COMPLETED) and done:
             return DoneAndNotDoneFutures(done, not_done)