CMFCPropertySheet Apply button disabled

Question

I created a CMFCPropertySheet derived class and added 2 CMFCPropertyPage derived classes using AddPage().

In both added page classes i've overridden the OnApply() method. When the pages are created and their OnCreate() is called i call SetModified(TRUE).

http://msdn.microsoft.com/en-us/library/aa299489(VS.60).aspx

I hoped the Apply button would be enabled, but it is still disabled. Does anyone know how to enable the Apply button for a CMFCPropertySheet ?