Rubber Fails Using XeLaTeX

My document builds just fine using xelatex, but now I am trying rubber and I get the following error:

\$ rubber --module xelatex proposal
Source path uses special characters, error tracking might get confused.
compiling proposal.tex...
/usr/local/texlive/2013/texmf-dist/tex/latex/fontspec/fontspec.sty:41: ..............................................


I have no idea how to debug this, since the error appears to be in fontspec.sty and not in my document itself.

I don't think I'm doing anything particularly weird in my preamble either:

\documentclass[letterpaper,11pt,oneside]{report}

\usepackage[T1]{fontenc}
\usepackage[final]{microtype}
\usepackage[margin=1.0in]{geometry}
\usepackage{graphicx}
\usepackage{fontspec}
\usepackage[backend=bibtex,url=false]{biblatex}
\usepackage{hyperref}
\usepackage{titlesec}
\usepackage{fancyhdr}
\usepackage{lipsum}

\pagestyle{fancy}

\renewcommand{\chaptername}{}
\setcounter{chapter}{0}
\titleformat{\chapter}{\normalfont\huge\bfseries}{\chaptertitlename\thechapter}{1em}{}
\titleclass{\chapter}{straight}

\setmainfont[Ligatures=TeX]{Minion Pro}

Nothing to do with the actual problem, but you should drop the usage of fontenc when compiling with xelatex and fontspec. – Johannes_B Dec 11 '13 at 14:14