Using `\tikzexternalize` generates a warning and an error for each tikzpicture

[tp971]

When using \tikzexternalize from the TikZ library "external", latexml generates a warning and an error for each tikzpicture in the document:

\documentclass{article}

\usepackage{tikz}
\usetikzlibrary{external}
\tikzexternalize

\begin{document}

\begin{tikzpicture}
    \node [draw=black] {Test};
\end{tikzpicture}

\end{document}

This document produces the following warning and error:

Warning:expected:<number> Missing number, treated as zero
        at test.tex; line 11 col 0 - line 11 col 17

Error:malformed:svg:foreignObject <svg:foreignObject> isn't allowed in <ltx:text>
        at test.tex; line 11 col 0 - line 11 col 17

However, the tikzpicture seems to be embedded correctly in the final HTML.