gpu ray sensor fails to scan vertically

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Hi I recently encountered a problem with the gpu ray sensor. when I create a sensor that has only horizontal scans, the ray detects the object in front of it, but when I add vertical scans it does not. When I change my sensor type from "gpu_ray" to "ray" the sensor works as expected (detects the object).

I'm currently working on gazebo 1.9.6, with Ubuntu 12.04.

I attached my XML.simple_test.world

What am I doing wrong?

thank you.

UPDATE: I also tried this on gazebo 2.2.3 with Ubuntu 14, and got the same result.