diff --git a/tasmota/tasmota_xdrv_driver/xdrv_39_thermostat.ino b/tasmota/tasmota_xdrv_driver/xdrv_39_thermostat.ino
index b3af31b20..ed6724485 100644
--- a/tasmota/tasmota_xdrv_driver/xdrv_39_thermostat.ino
+++ b/tasmota/tasmota_xdrv_driver/xdrv_39_thermostat.ino
@@ -1400,7 +1400,7 @@ void ThermostatGetLocalSensor(uint8_t ctr_output) {
         &&(ctr_output < THERMOSTAT_SENSOR_NUMBER)) {
       char temp[4];
       temp[0] = IndexSeparator();
-      snprintf(&temp[1], 4, "%u", (ctr_output + 1));
+      snprintf(&temp[1], sizeof(temp)-1, "%u", (ctr_output + 1));
       sensor_name.concat(temp);
     }
     JsonParserToken value_token = root[sensor_name].getObject()[PSTR("Temperature")];