How to display math operators not in italic?

I'm trying to write the following equation:

\rho = \max \left\{ \frac{|\mathbf{w} \cdot \mathbf{x^{(i)}} + b|}{\|\mathbf{w}\|} \right\} 

but the PDF output renders the math operator \max in italic instead of upright. I've tried the same with \log and \cos, and they behave just the same. What am I doing wrong here?

Document class: memoir

Packages: pgf, fontspec, graphicx, microtype, unicode-math, amsmath

The following code reproduces the problem in different systems:

\documentclass{memoir} \usepackage{unicode-math} \usepackage{amsmath} \begin{document} Yay, a dual problem: \begin{equation} \max_{\mathbf{\alpha}} \left\{ \sum_{i=1}^m \alpha{(i)} \right\} \end{equation} \end{document}