diff --git a/distributed/cfexecutor.py b/distributed/cfexecutor.py
index fda2fe63fb3..92031247c26 100644
--- a/distributed/cfexecutor.py
+++ b/distributed/cfexecutor.py
@@ -128,10 +128,9 @@ def map(self, fn, *iterables, **kwargs):
 
         fs = self._client.map(fn, *iterables, **self._kwargs)
 
-        if isinstance(fs, list):
-            # Below iterator relies on this being a generator to cancel
-            # remaining futures
-            fs = (val for val in fs)
+        # Below iterator relies on fs being an iterator itself, and not just an iterable
+        # (such as a list), in order to cancel remaining futures
+        fs = iter(fs)
 
         # Yield must be hidden in closure so that the tasks are submitted
         # before the first iterator value is required.
@@ -148,8 +147,7 @@ def result_iterator():
                         yield future.result()
             finally:
                 remaining = list(fs)
-                for future in remaining:
-                    self._futures.add(future)
+                self._futures.update(remaining)
                 self._client.cancel(remaining)
 
         return result_iterator()