diff --git a/micropython/net/ntptime/ntptime.py b/micropython/net/ntptime/ntptime.py
index 2402a6276..de161a2f0 100644
--- a/micropython/net/ntptime/ntptime.py
+++ b/micropython/net/ntptime/ntptime.py
@@ -22,6 +22,8 @@ def time():
     if len(msg) < 48:
         raise OSError(-1)
     val = struct.unpack("!I", msg[40:44])[0]
+    if not (msg[1] and val):
+        raise OSError(-1)
 
     # 2024-01-01 00:00:00 converted to an NTP timestamp
     MIN_NTP_TIMESTAMP = 3913056000
diff --git a/micropython/net/ntptime/test_ntptime.py b/micropython/net/ntptime/test_ntptime.py
index 2a868e824..acc83c06b 100644
--- a/micropython/net/ntptime/test_ntptime.py
+++ b/micropython/net/ntptime/test_ntptime.py
@@ -38,8 +38,9 @@ def socket(self, *a, **k):
 import ntptime
 
 
-def ntp_msg(ts=3913056000):
+def ntp_msg(ts=3913056000, stratum=2):
     msg = bytearray(48)
+    msg[1] = stratum
     struct.pack_into("!I", msg, 40, ts)
     return bytes(msg)
 
@@ -71,5 +72,31 @@ def test_time_short_raises():
         ntptime.socket = orig
 
 
+def test_time_kod_stratum_raises():
+    orig = _patch(ntp_msg(stratum=0))
+    try:
+        try:
+            ntptime.time()
+            assert False, "expected OSError"
+        except OSError as e:
+            assert e.args == (-1,)
+    finally:
+        ntptime.socket = orig
+
+
+def test_time_zero_ts_raises():
+    orig = _patch(ntp_msg(ts=0))
+    try:
+        try:
+            ntptime.time()
+            assert False, "expected OSError"
+        except OSError as e:
+            assert e.args == (-1,)
+    finally:
+        ntptime.socket = orig
+
+
 test_time_ok()
 test_time_short_raises()
+test_time_kod_stratum_raises()
+test_time_zero_ts_raises()