CMOS camera hardware trigger, Invalid_handle

Hi guys

I'm trying to use hardware trigger feature of thprlabs CMOS camera with this code:

NET.addAssembly('C:\Program Files\Thorlabs\Scientific Imaging\ThorCam\uc480DotNet.dll')
cam = uc480.Camera;
cam.Init(0);
cam.Trigger.Burst.Set(10)

ans = 
INVALID_HANDLE

where 'Trigger' is a property of 'uc480.Camera' and 'Burst' is a property of 'Trigger'.

Would you help me to find out what the problem is?

Thank you so much

Shabnam