# Angle of \ddots in an uneven matrix

I have the following triangle matrix with zeroes on the diagonal

\documentclass[a4paper]{memoir}
\usepackage{amsmath}
\begin{document}
$M = \begin{pmatrix} 0 & a_{1,2} & a_{1,3} & \dots & a_{1,n} \\ & 0 & a_{2,3} & \dots & a_{2,n} \\ & & 0 & \ddots & \vdots \\ & & & & 0 \end{pmatrix}$
\end{document}


The small problem I have now, is that the dots from \ddots point from a_{2,3} to the last zero. This could lead to the impression that the diagonal is not given by the zeroes. Anyone an idea?

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\documentclass[a4paper]{memoir}
$M = \begin{pmatrix} 0 & a_{1,2} & a_{1,3} & \dots & a_{1,n} \\ & 0 & a_{2,3} & \dots & a_{2,n} \\ & & 0 & \ddots & \vdots \\ & & & 0 & a_{n-1,n} \\ & & & & 0 \end{pmatrix}$