downcast to derived class in privately inherited base class templa

This unmanaged C++ code works with Visual Studio 2003 but raises "bad dynamic
cast exception" in Visual Studio 2005:

template< typename D >
class Base
{
public:
    virtual ~Base( ) { ; }
    void doFunc( )
    {
        D* dp = &dynamic_cast< D& >( *this ); // CRASH at runtime!
        // ... more code using "dp"
    }
};

class Derived : private Base< Derived >
{
    friend class Base< Derived >;
public:
    void doFunc( ) { Base< Derived >::doFunc( ); }
};

void test( )
{
    Derived d;
    d.doFunc( );
}

If Base<Derived> is inherited "public", it just works, if inherited
"protected", it will crash, too.
But for other reasons I can't inherit "public" in the original application's
code, it must be a "private" inheritance.
I want to use dynamic_cast to reference, because I want to have an
exception, if D is not derived from Base<D>.
Does anyone have an idea for a workaround, that only affects
Base<D>::doFunc() ?

Greetings from Cologne (Germany)
Georg Krichel