how come my IServiceProvider return null?

Hey let's say i got the MozillaWindowClass, i search the child of it
until i get a control ID of 1, then i use

HRESULT words = AccessibleObjectFromEvent( childHandler, OBJID_CARET,
CHILDID_SELF, &pAcc, &varChild );

finally i used

        IServiceProvider *pServProv = NULL;

        pAcc->QueryInterface(IID_IServiceProvider, (void**)&pServProv);

why is it pServProv returns null? I checked pAcc to be not null.