diff -r 8f72bf88f471 Lib/asyncio/futures.py
--- a/Lib/asyncio/futures.py	Sat Feb 20 12:59:36 2016 -0800
+++ b/Lib/asyncio/futures.py	Mon Feb 22 03:14:39 2016 +1100
@@ -341,6 +341,8 @@
             raise InvalidStateError('{}: {!r}'.format(self._state, self))
         if isinstance(exception, type):
             exception = exception()
+        if isinstance(exception, StopIteration):
+            raise TypeError("StopException interacts badly with generators and cannot be raised into a Future")
         self._exception = exception
         self._state = _FINISHED
         self._schedule_callbacks()
diff -r 8f72bf88f471 Lib/test/test_asyncio/test_futures.py
--- a/Lib/test/test_asyncio/test_futures.py	Sat Feb 20 12:59:36 2016 -0800
+++ b/Lib/test/test_asyncio/test_futures.py	Mon Feb 22 03:14:39 2016 +1100
@@ -76,6 +76,9 @@
         f = asyncio.Future(loop=self.loop)
         self.assertRaises(asyncio.InvalidStateError, f.exception)
 
+        # StopIteration cannot be raised into a Future - issue26221
+        self.assertRaises(TypeError, f.set_exception, StopIteration)
+
         f.set_exception(exc)
         self.assertFalse(f.cancelled())
         self.assertTrue(f.done())