diff --git a/ha_mqtt_discoverable/__init__.py b/ha_mqtt_discoverable/__init__.py
index 6098c35..57c0290 100644
--- a/ha_mqtt_discoverable/__init__.py
+++ b/ha_mqtt_discoverable/__init__.py
@@ -740,6 +740,7 @@ def _state_helper(
             topic = self.state_topic
         if last_reset:
             state = {"state": state, "last_reset": last_reset}
+            state = json.dumps(state)
         logger.debug(f"Writing '{state}' to {topic}")
 
         if self._settings.debug:
diff --git a/tests/test_sensor.py b/tests/test_sensor.py
index 6c5980a..7adc4ce 100644
--- a/tests/test_sensor.py
+++ b/tests/test_sensor.py
@@ -17,6 +17,7 @@
 
 from ha_mqtt_discoverable import Settings
 from ha_mqtt_discoverable.sensors import Sensor, SensorInfo
+from unittest.mock import patch
 
 
 @pytest.fixture(params=["°C", "kWh"])
@@ -46,4 +47,21 @@ def test_generate_config(sensor: Sensor):
 
 
 def test_update_state(sensor: Sensor):
-    sensor.set_state(1)
+    with patch.object(sensor.mqtt_client, "publish") as mock_publish:
+        sensor.set_state(1)
+        mock_publish.assert_called_with(sensor.state_topic, "1", retain=True)
+
+
+def test_update_state_with_last_reset(sensor: Sensor):
+    from datetime import datetime, timedelta, timezone
+
+    now = datetime.now(timezone(timedelta(hours=1)))
+    midnight = now.replace(hour=0, minute=0, second=0, microsecond=0)
+
+    with patch.object(sensor.mqtt_client, "publish") as mock_publish:
+        sensor.set_state(1, midnight.isoformat())
+        parameter = mock_publish.call_args.args[1]
+        import json
+
+        parameter_json = json.loads(parameter)
+        assert parameter_json["last_reset"] == midnight.isoformat()