RobotStudio event

Unable to retrieve 'ElapsedProductionTimeSinceLastService'

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Hello,

I have two identical controllers with the same RW 6.0.8 and I am trying to retrieve the value of ElapsedProductionTimeSinceLastService using the following syntax:

 _controller.MotionSystem.ActiveMechanicalUnit.ServiceInfo.ElapsedProductionTimeSinceLastService.TotalHours.ToString()

On one robot, it returns the number of hours worked, but on the other one, I get an error: "Specified argument was out of the range of valid values."

I also have another identical pair of robots, and it's the same - one returns hours, and the other returns the error "Index was outside the bounds of the array".

I am using PCSDK 2022.

Thank you for your help.