Selecting math font with unicode-math in beamer with XeLaTeX

I'm trying to use unicode-math in XeLaTeX to use Asana Math in a beamer presentation, but I am unable to use select the font for letters in math mode. See the following MWE:

\documentclass{beamer}
\usefonttheme{serif}

\usepackage{unicode-math}
\setmainfont{Linux Libertine O}
\setmathfont{Asana Math}
%\setmathfont[range=\mathit]{Asana Math} % has no effect

\begin{document}
\begin{frame}
Maths font test:
\begin{itemize}
\item math letters display in Libertine: $abcdefghijklmnopqrtuvwxyz$
\item math operators and blackboard letters display in Asana Math: $\int_{-\infty}^{+\infty} \sum \mathbb{E}$
\end{itemize}
\end{frame}
\end{document}


Which results in the following:

The letters are displayed in Linux Libertine, instead of Asana Math. I tried to tinker with the range option in unicode-math (as documented in the manual), to no avail.

Strangely, when I compile the document as an article, I get the results I expect. Am I missing something?

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\documentclass{beamer}