Problems compiling Tufte title page in XeLaTex

It's a bug in the tufte suite. The minimal example

\documentclass{tufte-book}

\title{this is a title}
\author{me}

\begin{document}

\maketitle

\end{document}

also fails when compiled with latex, because with latex and xelatex there's no possibility of using microtype for letterspacing, so tufte-common.def resorts to using soul features. However, \MakeTextUppercase fails with this method.

For XeLaTeX a workaround can be found in my answer to XeTeX seems to break headers in Tufte-handout

\documentclass{tufte-book}

\usepackage{ifxetex}
\ifxetex
  \newcommand{\textls}[2][5]{%
    \begingroup\addfontfeatures{LetterSpace=#1}#2\endgroup
  }
  \renewcommand{\allcapsspacing}[1]{\textls[15]{#1}}
  \renewcommand{\smallcapsspacing}[1]{\textls[10]{#1}}
  \renewcommand{\allcaps}[1]{\textls[15]{\MakeTextUppercase{#1}}}
  \renewcommand{\smallcaps}[1]{\smallcapsspacing{\scshape\MakeTextLowercase{#1}}}
  \renewcommand{\textsc}[1]{\smallcapsspacing{\textsmallcaps{#1}}}
  \usepackage{fontspec}
\fi

\title{this is a title}
\author{me}

\begin{document}

\maketitle

\end{document}

Of course you'll have to define font substitutes for the text fonts; the choice

\ifxetex
  \newcommand{\textls}[2][5]{%
    \begingroup\addfontfeatures{LetterSpace=#1}#2\endgroup
  }
  \renewcommand{\allcapsspacing}[1]{\textls[15]{#1}}
  \renewcommand{\smallcapsspacing}[1]{\textls[10]{#1}}
  \renewcommand{\allcaps}[1]{\textls[15]{\MakeTextUppercase{#1}}}
  \renewcommand{\smallcaps}[1]{\smallcapsspacing{\scshape\MakeTextLowercase{#1}}}
  \renewcommand{\textsc}[1]{\smallcapsspacing{\textsmallcaps{#1}}}
  \usepackage{fontspec}
  \setmainfont{TeX Gyre Pagella}
  \setsansfont{TeX Gyre Heros}[Scale=MatchUppercase]
\fi

should produce a quite similar output. The method for choosing the font may be different on your machines, it mostly depends on the installation; so mileage may vary. (Please, ask in comments if the proposed code doesn't work for you.)

Both MWE 1 and MWE 2 compile with pdfLaTeX. Neither MWE 1 nor MWE 2 compiles with XeLaTeX. This is at the command line - the editor is irrelevant.

The following MWE demonstrates the problem:

\documentclass{tufte-book}

\title{this is a title}
\author{me}

\begin{document}

\maketitle

\end{document}

The issue apparently has to do with \MakeTextUppercase which is triggering an error with \maketitle for XeLaTeX but not pdfLaTeX.

Just to clarify, \MakeTextUppercase is provided by textcase. However, I cannot reproduce the error with textcase alone so it has something to do with tufte's usage of it.