diff --git a/radicale/__main__.py b/radicale/__main__.py
index 991a66ef..41f567c3 100644
--- a/radicale/__main__.py
+++ b/radicale/__main__.py
@@ -142,7 +142,7 @@ def run():
 
     # SIGTERM and SIGINT (aka KeyboardInterrupt) shutdown the server
     def shutdown(signal_number, stack_frame):
-        shutdown_socket.sendall(b" ")
+        shutdown_socket.close()
     signal.signal(signal.SIGTERM, shutdown)
     signal.signal(signal.SIGINT, shutdown)
 
diff --git a/radicale/server.py b/radicale/server.py
index bd0e5d54..d0ca0d0b 100644
--- a/radicale/server.py
+++ b/radicale/server.py
@@ -84,7 +84,7 @@ else:
 class ParallelHTTPServer(ParallelizationMixIn,
                          wsgiref.simple_server.WSGIServer):
 
-    # wait for child processes/threads
+    # Python 3.6: Wait for child processes/threads (Default in Python >= 3.7)
     _block_on_close = True
 
     def __init__(self, configuration, address_family,
@@ -315,9 +315,9 @@ def serve(configuration, shutdown_socket=None):
         select_timeout = 1.0
     logger.info("Radicale server ready")
 
-    with contextlib.ExitStack() as stack:
+    with contextlib.ExitStack() as exit_stack:
         for _, server in servers.items():
-            stack.callback(server.server_close)
+            exit_stack.callback(server.server_close)
         while True:
             rlist, _, xlist = select.select(
                 sockets, [], sockets, select_timeout)