Why does Code Analysis in VS2013 RC lead me into a box canyon?

I ran Code Analysis on a utility I'm maintaining, and it advised me to change this:

private static extern int ReadMenu1File(string Menu1Path);

...to this:

private static extern int ReadMenu1File(UnmanagedType.LPWStr Menu1Path);

...with this verbiage: "Specify marshaling for P/Invoke string arguments To reduce security risk, marshal parameter 'Menu1Path' as Unicode, by setting DllImport.CharSet to CharSet.Unicode, or by explicitly marshaling the parameter as UnmanagedType.LPWStr. If you need to marshal this string as ANSI or system-dependent, specify MarshalAs explicitly, and set BestFitMapping=false; for added security, also set ThrowOnUnmappableChar=true."

...but when I did, it says, "The type name 'LPWStr' does not exist in the type 'System.Runtime.InteropServices.UnmanagedType'" and "'System.Runtime.InteropServices.UnmanagedType.LPWStr' is a 'field' but is used like a 'type'"

Code completion is not helping (no suggestions after typing "UnmanagedType.") nor is there a context menu option to add a missing using.