diff --git a/systemd_notifier.rb b/systemd_notifier.rb
index 995ab33..ad14e3e 100644
--- a/systemd_notifier.rb
+++ b/systemd_notifier.rb
@@ -27,7 +27,11 @@ class SystemdNotifier
   else
     MethodExecutor.record("debug", "heartbeat", "Found NOTIFY_SOCKET, enabling heartbeat")
     @enable_heartbeat = true
-    @socket = UNIXSocket.new(@notify_socket)
+    # Fix(2026-06-08): 将 UNIXSocket(SOCK_STREAM) 改为 SOCK_DGRAM
+    # systemd NOTIFY_SOCKET 是 datagram socket，用 stream socket 连接会报
+    #    Protocol wrong type for socket (Errno::EPROTOTYPE)
+    @socket = Socket.new(Socket::AF_UNIX, Socket::SOCK_DGRAM)
+    @socket.connect(Socket.pack_sockaddr_un(@notify_socket))
     MethodExecutor.record("debug", "heartbeat", "Heartbeat socket opened")
   end