Vista - DiInstallDevice with DIIDFLAG_INSTALLNULLDRIVER fails for Modem de

Hi,
installation of a NULL-driver for a device of class "Modem" fails with error
code 0xE0000203 (ERROR_NO_DRIVER_SELECTED). DiInstallDevice is called as
follows:

BOOL NeedReboot = FALSE;
if (!DiInstallDevice(
NULL, //HWND hwndParent OPTIONAL,
mHandle, //HDEVINFO DeviceInfoSet,
mDeviceInfoData //PSP_DEVINFO_DATA DeviceInfoData,
NULL, //PSP_DRVINFO_DATA
DriverInfoData,
DIIDFLAG_INSTALLNULLDRIVER, //DWORD Flags,
&NeedReboot //PBOOL NeedReboot OPTIONAL
) )
{ ... error handling ... }

The setupapi log file contains the following information:
cmd ...
dvi: {Plug and Play Service: Device Install for
USB\VID...&PID...&MI_02\...}
ump: Creating Install Process: DrvInst.exe 11:13:38.220
! ndv: Installing NULL driver!
dvi: Set selected driver complete.
pol: Device installation is subject to policy
dvi: {DIF_ALLOW_INSTALL} 11:13:38.236
dvi: Using exported function 'ClassInstall32' in module
'C:\Windows\system32\mdminst.dll'.
dvi: Class installer == mdminst.dll,ClassInstall32
dvi: No CoInstallers found
dvi: Class installer: Enter 11:13:38.251
!!! dvi: Class installer: failed(0xe0000203)!
!!! dvi: Error 0xe0000203: There is no driver selected for the
device information set or element.
dvi: {DIF_ALLOW_INSTALL - exit(0xe0000203)} 11:13:38.251
dvi: {DIF_DESTROYPRIVATEDATA} 11:13:38.251
dvi: Class installer: Enter 11:13:38.251
dvi: Class installer: Exit
dvi: {DIF_DESTROYPRIVATEDATA - exit(0x00000000)} 11:13:38.251
ump: Server install process exited with code 0xe0000203 11:13:38.251
ump: {Plug and Play Service: Device Install exit(e0000203)}
<<< Section end 2008/07/21 11:13:38.251
<<< [Exit status: FAILURE(0xe0000203)]

The problem occures under Vista Ultimate x86 and x64, with and without SP1.
With other setup classes there is no problem.

What's wrong and how can I solve this?

Best regards.