Template sensor causes "TypeError: unsupported operand type(s) for +: 'int' and 'NoneType'" during startup

I have a sensor that “computes” the average indoor temperature, based on the current_temperature attribute of all entities in the climate domain. However, it fails during startup, probably as some climate readings are not available.

So, in homeassistant’s log it prints.

2022-02-02 09:37:34 ERROR (MainThread) [homeassistant.components.template.template_entity] TemplateError(‘TypeError: unsupported operand type(s) for +: ‘int’ and ‘NoneType’’) while processing template ‘Template(“{{ ‘%2.1f’ | format(states.climate | map(attribute = ‘attributes.current_temperature’, default = 21.0) | sum / (states.climate | length) | float) }}”)’ for attribute ‘_attr_native_value’ in entity ‘sensor.indoor_temp_avg’

And this is the sensor’s configuration:

sensor:
  - platform: template                                                                         
    sensors:                                                                                       
      indoor_temp_avg:                                                                                       
        unit_of_measurement: '°C'                                                                
        value_template: "{{ '%2.1f' | format(states.climate | map(attribute = 'attributes.current_temperature', default = 21.0) | sum / (states.climate | length) | float) }}"

Anyway to avoid this by adding just another default value anywhere?