"Greek" language in Babel makes "index" fail for roman-numbered pages

Question

The following MWE works fine:

\documentclass{book}
\usepackage{makeidx}
% \usepackage[greek,english]{babel}
\makeindex
\begin{document}
\frontmatter
foo\index{foo}
\mainmatter
\printindex
\end{document}

But if I uncomment the \usepackage[greek,english]{babel} I get an error (even though the MWE doesn't contain any Greek text). My .idx file has changed from

\indexentry{foo}{i}

to

\indexentry{foo}{{\fontencoding  {OT1}\selectfont  i}}

and the .ilg file says

!! Input index error (file = test.idx, line = 1):
   -- Illegal space within numerals in second argument.

How can I get rid of the unwanted font encoding?

The document was compiled with latexmk (with the default options) in TexLive 2016.

Answer 1

Christian Hupfer's comment gives Does a current workaround for babel and indexing Roman-numeral page numbers exist? as an answer, but since I never want to use Roman numerals in Greek text, just restoring the LaTeX definition of \@roman after babel has "enhanced" it also works:

\documentclass{book}
\usepackage{makeidx}

\makeatletter
\let\save@roman\@roman
\usepackage[greek,english]{babel}
\let\@roman\save@roman
\makeatother

\makeindex
\begin{document}
\frontmatter
foo\index{foo}
\mainmatter
\printindex
\end{document}

The same idea would also work for \@Roman, of course.