Discrepancy of ray_sensor and gpu_ray sensor - horizontal ray scan

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Hi,

I have the following sensor description (code snippet!) in my robot description:

  <sensor name="my_laser_sensor" type="ray">
     <ray>
        <scan>
          <horizontal>
            <samples>640</samples>
            <resolution>1</resolution>
            <min_angle>0</min_angle>
            <max_angle>0</max_angle>
          </horizontal>
        </scan>
     </ray>
 </sensor>

This sensor works, I get 1 laser beam. Now I need to use a gpu_ray sensor instead of the ray_sensor, looking at http://sdformat.org/spec?ver=1.6&elem... I am using the default parameters for the ray scan. But when I try to change the type to gpu I get the following error:

EXCEPTION: Camera horizontal field of view invalid.

Even when I remove the horizontal code block, to use the default values by default the error remains.

Why is there a difference between the sensor definitions? How is it possible to simulate a single laser beam with the gpu_sensor?

Thanks!

Using gazebo 9.13