Problem including <AygShell.h>

I have a custom platform which includes "Standard SDK", "Standard
Shell" and "AYG Shell API Set" and am developing a small test app in
eVC++ using the SDK from the platform. I want to disable the OK button
on the app by using SHDoneButton() and so have included <AygShell.h> in
the .cpp file and linked with AygShell.lib.

When compiled the .cpp file gives the following error:

<etc>...include\mipsii\aygshell.h(68) : error C2061: syntax error :
identifier 'IShellPropSheetExt'

The line in question is a function prototype:

BOOL SHEnumPropSheetHandlers(HKEY hkey, int *pcPages, HPROPSHEETPAGE
*prghPropPages, IShellPropSheetExt **prgpispse);

It baffles me what the problem is because HPROPSHEETPAGE is defined in
prsht.h and IShellPropSheetExt in shlobj.h, both of which have
definately been included.

If I comment out this function prototype in aygshell.h (yes, I know I
shouldn't - it was only to see what happened), all works OK.

Any ideas????