Statistics sensor giving 'non-numerical' result

Configuration
1

I have a statistic sensor giving me ‘non-numerical’ result, although the input is numerical. Why?

This is the sensor:

sensor:

# create 'numeric values', the sensor data is not available when the inverter shuts down, the numeric values will then show 0
  - platform: template
    sensors:          
        sma_voltage_numeric:
            value_template: "{{ states('sensor.sma_voltage_l1') | float(0) | round(1) }}"
            unit_of_measurement: "V"
            unique_id: 52D8B4D81B


# calculate the 10 min average voltage at the inverter for the EV charger active overvoltage protection function
  - platform: statistics
    name: "Solar 10 min average voltage"
    entity_id: sensor.sma_voltage_numeric
    state_characteristic: mean
    max_age:
      minutes: 10
    precision: 1
    unique_id: E56FEBC536

I have a sensor for the SMA solar system voltage, which indeed goes ‘non-numerical’ when the sun is down. Therefore I create a numerical version of that same sensor and I use that for the statistics sensor. Still the statistic sensor goes ‘non-numerical’ when the voltage goes to zero.

The numerical version shows as zero on the dashboard, the statistics sensor shows an error on the dashboard (‘non-numerical’).