In version 4, I used to be able to write the equivalent of `\mathrm{d}^4 x` as `\dl^4 x`. However in version 5 I get and error about a "Missing { inserted". Is there a way to fix the problem?

MWE:

```
\documentclass[letterpaper,12pt]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage{diffcoeff}

\begin{document}
\begin{equation}
\dl^4 x
\end{equation}
\end{document}

```