Why this UnauthorizedAccessException?

Question

I don't understand why I get UnauthorizedAccessException on call to instantiate StreamWriter class.

string path = Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments);
using (StreamWriter sw = new StreamWriter(path, false ))
{
    // some processing here
}

I ran it from Visual Studio with VS been launched as Administrator. I then run application itself as Administrator. I tried it on a Windows 7 and a Windows 2012 R2 machine and same result. I feel like I am doing something silly here. Can any one point out what that may be?