Version of the custom_component
1.4.2 bb1cdba

Configuration

sensors:
# original sensor
  - platform: mqtt
    name: "Sonoff1 Temperature"
    state_topic: "tele/sonoff1/SENSOR"
    unit_of_measurement: '°C'
    value_template: "{{value_json['SI7021'].Temperature}}"

# average sensor
  - platform: average
    name: 'Sonoff1 Temperature Average'
    duration:
      days: 1
    entities:
      - sensor.sonoff1_temperature

# repeat the above with sonoff5 and sonoff6

Describe the bug

When the original sensor entity to compute the average of, is available but has an unknown state, this error occurs.
This might be related to #2?

To Reproduce

1. Unplug the temperature sensor. (Here i did that for the sonoff1 sensor which is the living room.)
2. The state of the original temperature sensor entity changes to unknown (see fig 1).
3. Create the average sensor as shown above and restart HA
   (So during the first start of the average sensor, the original sensor has no value, but there are still values in the period (1 day) to compute the average of, see fig 2)
4. The average sensor produces that error and does not compute the average value (see fig 3).
5. Even after plugging the sensor back in so it transmits data to HA and the original sensor entity has a float value again (see living room temp in fig 2 at the right side), the average sensor still throws that error.

The other average sensors (sonoff5 and sonoff6) worked fine, as they had a not-unknown state for their original sensors for > 1 day.

Expected behavior

I expected the average sensor to produce an average value of the configured period of all the not-unknown values in that period. It looks like, the average sensor cannot cope with past (or still ongoing) sections in the state history that have unknown values.

Screenshots

fig 1

fig 2

fig 3

Debug log

Update for sensor.sonoff1_temperature_average fails
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/entity.py", line 279, in async_update_ha_state
    await self.async_device_update()
  File "/usr/src/homeassistant/homeassistant/helpers/entity.py", line 472, in async_device_update
    await self.hass.async_add_executor_job(self.update)
  File "/usr/local/lib/python3.7/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/usr/src/homeassistant/homeassistant/util/__init__.py", line 240, in wrapper
    result = method(*args, **kwargs)
  File "/config/custom_components/average/sensor.py", line 284, in update
    self._update_state()
  File "/config/custom_components/average/sensor.py", line 472, in _update_state
    current_state = self._get_entity_state(item)
  File "/config/custom_components/average/sensor.py", line 260, in _get_entity_state
    state = self._get_temperature(entity) if self._temperature_mode \
  File "/config/custom_components/average/sensor.py", line 248, in _get_temperature
    temperature = float(temperature)
ValueError: could not convert string to float: 'None'

Thanks for your work at this component! Feel free to let me know how to further assist you in tracing this down.