Invalid conversion from utf8 string when it is in variable

[rsk82]

The effect here is that the message text is correct in greek 
letters but message title is all empty boxes, sometimes with 
other texts there are chinese chars.

string msg= "ένα μήνυμα";
MessageBoxW(null, "ένα μήνυμα", cast(wchar*)msg.toStringz, 
MB_ICONERROR);

http://s9.postimage.org/66e84i80r/msgbox.png