Erratic TInterpreter::GetInterfaceMethod behaviour?

I'm seeing a little erratic GetInterfaceMethod behaviour between my compiled program and running in root direct, and I don't know how to track it down. Perhaps there is some interpreter initialisation I need to do first? I've attached a test script that writes out a basic .C macro, then attempts to load it and read the address of a function out of it with the prescription:

  gROOT->LoadMacro(filename);
  TInterpreter* interp = gROOT->GetInterpreter();   void* func = interp->GetInterfaceMethod(0,funcname,"");   return (bool)func;

Running through root gives:
$ root ReadC.C

Result of search: 1

Running through ACLIC gives:
$ root ReadC.C+

Result of search: 1

But compiling as a stand-alone program gives a different answer:
$ g++ -o ReadC ReadC.C `root-config --cflags --libs`
$ ./ReadC

Failed to load function
Result of search: 0

Unfortunately, getting this to work is a necessity, as a pretty ingrained part of our codebase relies on this behaviour (most people run through rootscripts). Does anyone know what is causing this? I tested on 5.27/02 and 5.28/00 and both exhibited this behaviour.

Thanks,

Nick