Use the debugger to debug remote subprogram

In Natural Studio, I wrote a test program to callnat a subprogram in a remote UNIX box where NDV is already installed. The result is normal without debugger.

When tried using debugger, and at the point of stepping into the remote subprogram, it prompted :

“No source available for RPCSTEP”
and then
“No symbolic information for RPCSTEP”

I continously pressing ‘F7’ (ie. ‘step in’), eventually I still couldn’t see the remote subprogram which I wanted to debug! Though, the execution was complete with normal end result.

I know RPCSTEP is an object in SYSTEM library. But no matter what it is, does anyone know how can I debug my remote subprogram? Kindly advise please.