Oldstyle (arabic) numbers in headings remove roman pagination

Question

Using the following code to get oldstyle (arabic) numbers in the headings make the preamble page numbers (in roman) to disappear. Here's a minimal working example:

\documentclass{book}
\usepackage{fancyhdr}
\usepackage{lmodern}
\usepackage{lipsum}
\usepackage{multicol}
\usepackage{textcomp}
\usepackage{pdfpages}

\pagestyle{fancy}
\fancyhead[LE]{\oldstylenums{\thepage}}
\fancyhead[RO]{\oldstylenums{\thepage}}

\begin{document}
\frontmatter
\lipsum
\mainmatter
\lipsum
\end{document}

As you can see, in the frontmatter, there is no pagination on the headings (although there's still pagination at the bottom). Thanks.

Answer 1

\makeatletter
\fancyhead[LE]{\if@mainmatter\oldstylenums{\arabic{page}}\else\thepage\fi}
\fancyhead[RO]{\if@mainmatter\oldstylenums{\arabic{page}}\else\thepage\fi}
\makeatother

Answer 2

\thepage gives a roman number in the frontmatter. \oldstylenums doesn't know what to do with it. If you want a number in the header use \fancyhead[LE]{\oldstylenums{\arabic{page}}}.

Answer 3

\def\isnum#1{%
  \if!\ifnum9<1#1!\else_\fi
    \oldstylenums{#1}\else#1\fi}

\fancyhead[LE]{\isnum{\thepage}}}
\fancyhead[RO]{\isnum{\thepage}}}