diff --git a/distributed/core.py b/distributed/core.py
index 250ea076e03..16c611142f4 100644
--- a/distributed/core.py
+++ b/distributed/core.py
@@ -606,7 +606,7 @@ def close(self):
                 break
             else:
                 yield asyncio.sleep(0.05)
-        yield [comm.close() for comm in self._comms]  # then forcefully close
+        yield [comm.close() for comm in list(self._comms)]  # then forcefully close
         for cb in self._ongoing_coroutines:
             cb.cancel()
         for i in range(10):