I've created a thermometer sensor using an ESP8166 with MQTT to publish temperature updates. The JSON emitted is in the format:

```
{
    "temperature": "53.4"
}

```

In Home Assistant, I created a sensor in configuration.yaml:

```
sensor:
  - platform: mqtt
    name: Backyard Temperature
    unique_id: a1234
    state_topic: home/temperature
    value_template: '{{ value_json.temperature }}'
    unit_of_measurement: F

```

The sensor is correctly sending the JSON payload, and MQTT subscriptions running on my desktop are receiving the updates, but the value is never updated on the Home Assistant dashboard (the state remains as the literal "Unknown").

Here's what the rendered group looks like. Odd there's a switch on the group heading.
[![Group rendering](https://i.stack.imgur.com/AQJIG.png)](https://i.stack.imgur.com/AQJIG.png)

Can anyone see what I'm missing? According to the documentation, I've done everything for this to work.