How can I avoid the use of default styles when no listings language is set?

When global listings styles have been defined identifiers are coloured differently from numbers even if no language is in effect. How can this be avoided?

\documentclass{minimal}\usepackage{listings}\usepackage{xcolor}\lstset{identifierstyle=\color{purple}}\begin{document}\lstset{language=}\begin{lstlisting}123 hello world 456\end{lstlisting}\end{document}