[pstricks] Example 15.1.10 from PSTricks Book

[Hernando Diaz]

Trying to understand example 15.1.10 from Herbert's book, I ran into the
following error:
! Missing number. Treated as zero
When using month's name as label. The definitions \Monat and \Level work
fine. Could someone point my errors?

I am using the online version. 
Maybe I am missing something, because of the language barrier. My german
skills range from lousy to nonexisting.

The code I am using:
\listfiles
\documentclass{article}
\usepackage[latin1]{inputenc}
\usepackage{pstricks,pst-plot,pstricks-add}
\def\Monat #1{%
	\ifcase#1\or Januar\or Februar\or März\or April\or Mai\or Juni\or%
	Juli\or August\or September\or Oktober\or November\or Dezember\fi%
}%
\def\Level#1{\ifcase#1\or Wenig\or Mittel\or Viel\fi}%
\def\psvlabel#1{\footnotesize\Level{#1}}%
\def\pshlabel#1{\rput[rb]{30}{\footnotesize\Monat{#1}}}%
\begin{document}
\psset{unit=0.8}
Month: \Monat{3}, level: \Level{1} 
\begin{center}
	\begin{pspicture}(0.5,1)(13,4)
	\psaxes[showorigin=false]{->}(13,4)
	\end{pspicture}
\end{center}
\end{document}

*File List*
 article.cls    2005/09/16 v1.4f Standard LaTeX document class
  size10.clo    2005/09/16 v1.4f Standard LaTeX file (size option)
inputenc.sty    2006/03/04 v1.1b Input encoding file
  latin1.def    2006/03/04 v1.1b Input encoding file
arrayjob.sty    2000/05/03 v1.03 Array management
pstricks.sty    2005/12/19 v0.31 LaTeX wrapper for `PSTricks' (RN,HV)
pstricks.tex    2006/02/06 v1.14a `PSTricks' (tvz)
  xcolor.sty    2005/12/21 v2.09 LaTeX color extensions (UK)
   color.cfg    2005/12/29 v1.1 MiKTeX 'color' configuration
   dvips.def    1999/02/16 v3.0i Driver-dependant file (DPC,SPQR)
pst-plot.sty    2004/07/15 package wrapper for pst-plot.tex
pst-plot.tex    2004/05/18 1.41 `pst-plot' (tvz)
pstricks-add.sty    2005/03/03 package wrapper for pstricks-add.tex (hv)
pstricks-add.tex    2005/12/28 v2.73 `PSTricks-add' (hv)
pst-xkey.tex    2005/11/25 v1.6 PSTricks specialization of xkeyval (HA)
 xkeyval.sty    2005/11/25 v2.5e package option processing (HA)
 xkeyval.tex    2005/11/25 v2.5e key=value parser (HA)
 ***********