diff --git a/remotefilelog/fileserverclient.py b/remotefilelog/fileserverclient.py
--- a/remotefilelog/fileserverclient.py
+++ b/remotefilelog/fileserverclient.py
@@ -224,8 +224,9 @@
                     'node': m[-40:]
                 }))
 
-            for m, future in itertools.izip(missed, futures):
-                r = future.result()
+            for i, m in enumerate(missed):
+                r = futures[i].result()
+                futures[i] = None  # release memory
                 file_ = idmap[m]
                 node = m[-40:]
                 receivemissing(io.BytesIO('%d\n%s' % (len(r), r)), file_, node)