How to print crop marks with KOMA-Script class scrbook

Question

I want to print crop marks for a format smaller than a4 which is printed on a4 stock. I use KOMA-Script class scrbook and add geometry and crop packages - nothing happens. How can I have crop marks printed around the text so I can judge the layout?

\documentclass{scrbook}
\KOMAoptions{paper=      
    128.5mm:198.4mm, %(5,06" x 7,91")  %ziel
    BCOR=8mm,twoside,
    headinclude=false, footinclude=false, 
    headings=normal,
    titlepage=true,
    DIV=9,  %ziel kleines buch 
    fontsize=12pt,
}

\usepackage{geometry}
\usepackage[axes,cross,pdftex,center]{crop}

\begin{document}

some text 

\end{document}

Answer 1

Just add option a4 to tell package crop to use paper a4 to show crop informations and the little book.

Please see the following MWE

\documentclass{scrbook}
\KOMAoptions{%
  paper=128.5mm:198.4mm, %(5,06" x 7,91")  %ziel
    BCOR=8mm,twoside,
    headinclude=false, footinclude=false, 
    headings=normal,
    titlepage=true,
    DIV=9,  %ziel kleines buch 
    fontsize=12pt,
}

\usepackage{geometry}
\usepackage[%
  a4, % <===============================================================
  axes,cross,pdftex,center
]{crop}

\begin{document}

some text 

\end{document}

and its result: