Connect device without logical name using pos.net

See more:

POS.NET

I need to connect devices(Cash drawer,Printer) without logical name using pos.net. I am using below code
DeviceInfo POSDevice = posExplorer.GetDevice(DeviceType.CashDrawer);

It show error 'object reference not set to be instance of an object'

when i sued the below code then it works fine

DeviceInfo POSDevice = posExplorer.GetDevice(DeviceType.CashDrawer, "CD1_1");

Please let me know how i can rectify this problem.