CString::MakeUpper fails

Hi,
i´ve created a new mfc application sample project with VS2015 Update 3.
I used the default settings of Visual Studio and added these 3 lines in CMFCApplication1Dlg::OnInitDialog()

CString XXX = L"abc äöüß ABC ÄÖÜß";
XXX.MakeUpper();
AfxMessageBox(XXX);

Why do i get "ABC äöüß ABC ÄÖÜß" as return of MakeUpper.

It should be "ABC ÄÖÜß ABC ÄÖÜß"

Whats wrong?