strut and strutbox

The definition of a \strut in LaTeX is given in the source as:

211 \newbox\strutbox
212 \def\strut{\relax\ifmmode\copy\strutbox\else\unhcopy\strutbox\fi}

Shoudn't a \strutbox, give an empty box? AS you can see from the example below, it prints the ligature ae, if I include fixltx2e and the ligature ff if I don't. \strut works as expected.

\documentclass[11pt]{article}
\usepackage{fixltx2e}
\begin{document}
\strutbox
\texttt{\strutbox}
\textit{\strutbox}
\end{document}
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