! Package xkeyval Error: `[' undefined in families `background'.

documentclass{article}

% usepackage{xkeyval}
% usepackage{pax}
% usepackage{pdfpages}
%
% makeatletter
% define@key{PAX@Gin}{scale}{}
% makeatother

usepackage{background}
backgroundsetup[contents=preliminary,placement=bottom,color=blue]

usepackage{lipsum}
begin{document}
lipsum[1-10]
end{document}

pdfTeX 3.1415926-2.5-1.40.14 (TeX Live 2013)

backgroundsetup{contents=preliminary,placement=bottom,color=blue}

documentclass{article}

usepackage{background}
backgroundsetup{contents=preliminary,placement=bottom,color=blue}

usepackage{lipsum}
begin{document}
lipsum[1-10]
end{document}