diff --git a/pyhilo/device/__init__.py b/pyhilo/device/__init__.py
index 026b5b2..b08536d 100644
--- a/pyhilo/device/__init__.py
+++ b/pyhilo/device/__init__.py
@@ -94,6 +94,16 @@ def update(self, **kwargs: Dict[str, Union[str, int, Dict]]) -> None:
                 elif att == "model_number":
                     att = "model"
                 new_val = val  # type: ignore
+                if att == "identifier" and new_val is None:
+                    # The cloud returns a null serial for some devices (observed
+                    # for the Hilo gateway since ~2026-09-10). Consumers build
+                    # unique ids as f"{device.identifier.lower()}-..." and crash
+                    # on None, so coerce back to the historic empty-string value
+                    # that existing device registries were built from.
+                    # mypy: the declared kwargs type excludes None, but the
+                    # cloud really does send nulls — which is exactly the case
+                    # being guarded here, hence the ignore below.
+                    new_val = ""  # type: ignore[unreachable]
             setattr(self, att, new_val)
         if self.model:
             self.model = self.model.replace("Model_", "")
diff --git a/tests/test_device.py b/tests/test_device.py
index 36a824a..cdf5d27 100644
--- a/tests/test_device.py
+++ b/tests/test_device.py
@@ -1,10 +1,13 @@
-from datetime import datetime, timezone
+from datetime import datetime
 from unittest.mock import MagicMock
 
-import pytest
-
 from pyhilo.const import HILO_READING_TYPES
-from pyhilo.device import DeviceAttribute, DeviceReading, HiloDevice, get_device_attributes
+from pyhilo.device import (
+    DeviceAttribute,
+    DeviceReading,
+    HiloDevice,
+    get_device_attributes,
+)
 
 
 class TestDeviceAttribute:
@@ -97,7 +100,9 @@ def test_equality_by_attr_name(self):
 
     def test_inequality_different_attr(self):
         r1 = self._make_reading(device_attribute=DeviceAttribute("Power", "Watt"))
-        r2 = self._make_reading(device_attribute=DeviceAttribute("Intensity", "Percentage"))
+        r2 = self._make_reading(
+            device_attribute=DeviceAttribute("Intensity", "Percentage")
+        )
         assert r1 != r2
 
     def test_repr(self):
@@ -227,13 +232,19 @@ def test_update_readings_replaces_existing(self):
         d = HiloDevice(api, id=1)
         da = DeviceAttribute("Power", "Watt")
         r1 = DeviceReading(
-            deviceId=1, hiloId="h-1", locationId=10,
-            timeStampUTC="2024-01-15T10:30:00Z", value=100.0,
+            deviceId=1,
+            hiloId="h-1",
+            locationId=10,
+            timeStampUTC="2024-01-15T10:30:00Z",
+            value=100.0,
             device_attribute=da,
         )
         r2 = DeviceReading(
-            deviceId=1, hiloId="h-1", locationId=10,
-            timeStampUTC="2024-01-15T11:00:00Z", value=200.0,
+            deviceId=1,
+            hiloId="h-1",
+            locationId=10,
+            timeStampUTC="2024-01-15T11:00:00Z",
+            value=200.0,
             device_attribute=da,
         )
         d.update_readings(r1)
@@ -325,3 +336,11 @@ def test_get_attribute_unknown(self):
         d = HiloDevice(api, id=1)
         result = d.get_attribute("nonexistent")
         assert result is None
+
+    def test_identifier_none_coerces_to_empty_string(self):
+        """The cloud returns a null identifier for some devices (observed for
+        the Hilo gateway since ~2026-09-10); consumers build unique ids from
+        device.identifier and must not receive None."""
+        api = _mock_api()
+        d = HiloDevice(api, id=1, identifier=None)
+        assert d.identifier == ""