File Opener error in Debugger for Windows 8.1

Whenever I use the following code, everything executes fine, UNLESS I'm running in the debugger on a Windows 8.1 machine. Debugger works fine in Windows 7 and File Opener window works fine when running the executable directly (i.e. not through the debugger). Symptom of the crash is that a blank dialog box is displayed where the file selection window should be and an error about 'user32.dll' is displayed in the debugger window.

  I = winio@ ('%ca[Select Master File]&')
  I = winio@ ('%mn[&File[&Open]]&', 'file_openr',
 1                                             File_Name, Disp_Name)
  I = winio@ ('%mn[[E&xit]]', 'EXIT')

Thanks.

I have logged this for investigation.