[XeTeX] polyglossia problem Miktex 2.9

[Ahmad Essam]

hello,
I've some code that compiled successfully in previous version with XeLaTeX (Miktex 2.8) , but now the compilation produce this error:

Package polyglossia Error: 
The current roman font does not contain the Arabic script!
Please define \arabicfont with \newfontfamily.
The code that i used:

\documentclass[a4paper,twoside,headsepline,10pt]{scrbook}

\usepackage{polyglossia}

\setmainlanguage{arabic}
\setotherlanguage{english}
\setmainfont[Script=Arabic,Scale=1.5,Mapping=arabicdigits]{Traditional Arabic}

\begin{document}

\chapter{المقدمة}

%arabic text here...
السلام عليكم ورحمة الله وبركاته

\end{document}
I try to put this line in preamble:
\newfontfamily\arabicfont[Script=Arabic,Scale=1.5,Mapping=arabicdigits]{Traditional Arabic}

but it doesn't help.


How i can fix it ?
Thanks in advance.

 		 	   		  
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