Missing { inserted: \DeclarePairedDelimiterX

Question

The following code

\documentclass{scrartcl}
\usepackage{mathtools}
\DeclarePairedDelimiterX\innerp[2]{\left(}{\right)}{#1 \delimsize\vert \mathopen{}#2}
\DeclarePairedDelimiterX{\dualp}[2]{\langle}{\rangle}{#1\, , \mathopen{}#2}
\begin{document}
%
\[
    \innerp{a}{b}
\]
%
%
\[
    \dualp{a}{b}
\]
%
\end{document}

Gives the following error code:

./mwe.tex:8: Missing { inserted.
<to be read again> 
                   \left 
l.8     \innerp{a}{b}

I have just replaced the \langle by \left( and the \rangle by \right) . So I am wondering what is wrong.