Error using longtable and xwatermark

Question

I am experiencing some issues when I use longtable and xwatermark.

If the table starts in page X and finishes two page later, I get an error : ! LaTeX Error: Something's wrong--perhaps a missing \item And it points right at the end of the table.

I think it is xwatermark that adds something whitin each page that annoys pdftex.

If I ignore the error, the file is generated anyway. But I would like to have no error to be able to debug future version easily (I am beggining with LaTeX).

\documentclass[12pt,a4paper]{book}

\usepackage[english]{babel}
\usepackage{pdfpages}
\usepackage[textheight=2cm]{geometry}
\usepackage{longtable}
\usepackage{xwatermark}

\newwatermark[oddpages,fontfamily=,color=gray!25,angle=-90,scale=0.5,xpos=0.45\paperwidth,ypos=0]{\textsc{Confidential}}
\newwatermark[evenpages,fontfamily=,color=gray!25,angle=90,scale=0.5,xpos=-0.45\paperwidth,ypos=0]{\textsc{Confidential}}

\raggedbottom %to prevent Underfull \vbox (badness 10000) with the graphics

\makeatother

\begin{document}
\begin{center}
    \begin{longtable}{|c|c|}
        \hline
        a & a \\
        \hline
        b & b \\
        \hline
        c & c \\
        \hline
        d & d \\
        \hline
        e & e \\
        \hline
        f & f \\
        \hline
    \end{longtable}
\end{center}
\end{document}

Answer 1

The longtable is already centered by default. Remove \begin{center} and \end{center} and you are bailed out.