Overflow on \readindex command.

I have this sequence of code:

 \vfil\eject
\chapter{Index}

\readindexfile{i}

 \bye

I get this error message:
----------------------------------------------
(./book.ind
! Arithmetic overflow.
\ <at> columns ...iply \count \footins by \ <at> numcolumns
                                                  \advance \vsize by -\ht 
\t...

\ <at> beginindex ...ubitem \indexfonts \doublecolumns
                                                  \parindent = 0pt \hookrun 
...
l.1 \begin{theindex}
---------------------------
The file is in \singlecolumn mode at this point. \Chapter is defined as:
-------------------------------
\def\chapter#1{%
\rm             
\writetocentry{chapter}{#1}
\pdfoutline goto page \the\pageno {/Fit} {#1}
\leftline{\hbox{\medfont #1\rm}}
}