[XeTeX] how to escape from eternal loop of XeTeXinterchartoks?

[Dohyun Kim]

Hi,

\XeTeXinterchartokenstate=1
\XeTeXinterchartoks 0 0 {\futurelet\lastchar\dosomething}
\XeTeXinterchartoks 255 0 {\futurelet\lastchar\relax}
\def\dosomething{}
ABC \lastchar
\bye

This tex code falls into eternal loop and never reaches to \dosomething.
Does anybody know how to escape from this loop?
or is there another way to identify last char code?
Thanks in advance.
-- 
Dohyun Kim