** WARNING ** Failed to convert input string to UTF16...

[GoogleCodeExporter]

I've been receiving this warnings every time I compile a document with 
tufte-latex:

** WARNING ** Failed to convert input string to UTF16...

Nothing happens, the document compiles fine, but what do those warnings mean?

Here is my preamble in case it helps:

\documentclass[nols,xetex]{tufte-book}
\usepackage{fontspec}
\usepackage{xltxtra}
\usepackage{xunicode}%fontspec,xltxtra,
\defaultfontfeatures{Mapping=tex-text}
\setromanfont[Mapping=tex-text,Numbers=OldStyle]{SabonNext LT}
\setsansfont[Mapping=tex-text]{SabonNext LT}
\setmonofont[Scale=MatchLowercase]{Droid Sans}
\usepackage{polyglossia}
\setmainlanguage{brazil}
\usepackage{graphicx}
\usepackage{chngcntr} %isso aqui tira figura 0.1... etc
%\counterwithin*{footnote}{chapter}% reset the sidenote counter with each new 
chapter (doesn't work...)
\counterwithout{figure}{chapter}
\usepackage{eulervm}
\usepackage{xcolor}
\usepackage{hyperref}
%\setkomafont{disposition}{\sffamily}

\title{XXX}
\author{XXX}
\date{XXX

 \renewcommand{\allcapsspacing}[1]{{\addfontfeature{LetterSpace=20.0}#1}}
      \renewcommand{\smallcapsspacing}[1]{{\addfontfeature{LetterSpace=5.0}#1}}
      \renewcommand{\textsc}[1]{\smallcapsspacing{\textsmallcaps{#1}}}
      \renewcommand{\smallcaps}[1]{\smallcapsspacing{\scshape\MakeTextLowercase{#1}}}

\begin{document}

...

Original issue reported on code.google.com by yche...@gmail.com on 7 Dec 2011 at 11:46