Sensor for total electric energy combined LOW + HIGH


I want to use a new sensor (energie_verbruik_totaal) to display on the dashboard as gauge meter.
This sensor needs to combine the Low and High energy current consumption
low is available in the evening and weekend
high is available monday to friday from 8.00-23:00

Sometimes the code below works and somtenies not , could somebody help ?

error on dashboard is: entity is not numeric sensor energie_verbruik_totaal

code in confirguration.yaml:

platform: template
        friendly_name: "Energie Verbruik (totaal)"
        unit_of_measurement: 'W'
        value_template: "{{states('sensor.toon_p1_power_use_low') | float(0)+ states('sensor.toon_p1_power_use_high') | float(0) }}"


Thanks in advance.