C# .NET marshaling on Ubuntu Linux 14.04

Would anyone know why the following C# marshaiing code might return an erroneous return value?

var insertCommand =  "INSERT INTO message ( text ) VALUES (\"HelloWorld\")";

public static IntPtr StringToHGlobalUTF8(string s, out int length)
        {
            if (s == null)
            {
                length = 0;
                return IntPtr.Zero;
            }

            var bytes = Encoding.UTF8.GetBytes(s);
            var ptr = Marshal.AllocHGlobal(bytes.Length + 1);
            Marshal.Copy(bytes, 0, ptr, bytes.Length);
            Marshal.WriteByte(ptr, bytes.Length, 0);
            length = bytes.Length;

            return ptr;
        }

any help is greatly  appreciated.