Why wont this template work?

Created atolls section with the custom-ui , I have a set of tiles with heights that adjust to the info being displayed dynamically. I want to have the tiles only show 2 lines (instead of possible 3 when the value is too long to fit and needs the 3d row) because the image resizing causes problems. pleas have a look at this code

example:

  - entity: switch.sw_tester_template
    label_sec_template: >
      if (state === 'on') return 'Tester' + 
            entities['sensor.tester_actueel'].state;
      return 'Tester off';
    icon_template: >
      if (state === 'on') return 'mdi:test-tube';
      return 'mdi:test-tube-off';

this shows the Tester tile with name Tester and its value.

Trying to take the word Tester out only to show the value, I tested this:

  - entity: switch.sw_tester_template
    label_sec_template: >
      if (state === 'on') return   entities['sensor.tester_actueel'].state;
      return 'Tester off';
    icon_template: >
      if (state === 'on') return 'mdi:test-tube';
      return 'mdi:test-tube-off';

But that wouldn’t show any text nor value. This however does work:

  - entity: switch.sw_tester_template
    label_sec_template: >
      if (state === 'on') return '' + 
            entities['sensor.tester_actueel'].state;
      return 'Tester off';
    icon_template: >
      if (state === 'on') return 'mdi:test-tube';
      return 'mdi:test-tube-off';

Why do i need to show the empty text ‘’ and doesnt the entities[‘sensor.tester_actueel’].state (also tried states.sensor.tester_actueel.state ) show when on its own?

Thanks for you help,
Marius