diff --git a/src/websockets/sync/connection.py b/src/websockets/sync/connection.py
index e7807324..06ea00ef 100644
--- a/src/websockets/sync/connection.py
+++ b/src/websockets/sync/connection.py
@@ -923,8 +923,9 @@ def close_socket(self) -> None:
 
         # Calling protocol.receive_eof() is safe because it's idempotent.
         # This guarantees that the protocol state becomes CLOSED.
-        self.protocol.receive_eof()
-        assert self.protocol.state is CLOSED
+        with self.protocol_mutex:
+            self.protocol.receive_eof()
+            assert self.protocol.state is CLOSED
 
         # Abort recv() with a ConnectionClosed exception.
         self.recv_messages.close()
diff --git a/tests/sync/test_client.py b/tests/sync/test_client.py
index 7d817051..7ab8f4dd 100644
--- a/tests/sync/test_client.py
+++ b/tests/sync/test_client.py
@@ -151,7 +151,8 @@ def test_connection_closed_during_handshake(self):
         """Client reads EOF before receiving handshake response from server."""
 
         def close_connection(self, request):
-            self.close_socket()
+            self.socket.shutdown(socket.SHUT_RDWR)
+            self.socket.close()
 
         with run_server(process_request=close_connection) as server:
             with self.assertRaises(InvalidMessage) as raised: