diff --git a/netx_api/ume_alarm_ws_log.py b/netx_api/ume_alarm_ws_log.py
index 2506573..1538a9f 100644
--- a/netx_api/ume_alarm_ws_log.py
+++ b/netx_api/ume_alarm_ws_log.py
@@ -24,8 +24,11 @@ _SUBSCRIPTION_MISSING_MARKERS: tuple[str, ...] = (
     "status-reason: subscription not found",
     "non-101 status: 503",
     "non-101 status: 404",
+    "non-101 status: 500",
     "handshake status 404",
     "handshake status 503",
+    # ZTE UME often returns HTML 500 when the stream id is gone/invalid.
+    "handshake status 500",
 )
 
 
diff --git a/tests/test_ume_sync.py b/tests/test_ume_sync.py
index c048405..b5283e6 100644
--- a/tests/test_ume_sync.py
+++ b/tests/test_ume_sync.py
@@ -753,6 +753,11 @@ class UmeAlarmNotificationTests(unittest.TestCase):
                 'ume_request_failed:DELETE ... 400:{"errorInfo":"subscription not exist!"}'
             )
         )
+        self.assertTrue(
+            is_ume_subscription_missing_error(
+                "Handshake status 500 Server Error -+-+- {'content-type': 'text/html'}"
+            )
+        )
 
     def test_cancel_returns_needs_cleanup_when_ume_missing(self):
         from time import time as _time