# Undefined Macro in Package xstring

I have a very basic problem. I am trying to use the xstring package in a Latex document and I keep getting the following error message for an undefined control sequence:

(c:/Program Files (x86)/PCTeX/PCTeXv6/texmf-dist/tex/generic/xstring/xstring.sty

(c:/Program Files (x86)/PCTeX/PCTeXv6/texmf-dist/tex/generic/xstring/xstring.tex

****! Undefined control sequence.**

**\@xs@buildlines ...\expandafter {\number \numexpr
#1+1-1}\edef \@xs@reserved...**

**l.427 \@xs@newmacro\StrRemoveBraces{}{1}{1}
{%******

Typesetter process aborted

I downloaded and installed this package from CTAN; it is the 2013-07-29 version.