Use \ae ligature in bibliography [duplicate]

Question

I have a reference containing a name: Hornbaek. The author's used a ligature when writing their name. The citation uses Hornb\aek, which doesn't work in the bibliography, but does work in text. Here's an example:

\documentclass{article}

\usepackage{filecontents}

\begin{filecontents}{\jobname.bib}
@inproceedings{Rasmussen:2012:SIR:2207676.2207781,
 author = {Rasmussen, Majken K. and Pedersen, Esben W. and Petersen, Marianne G. and Hornb\aek, Kasper},
 title = {Shape-changing interfaces: a review of the design space and open research questions},
 booktitle = {Proceedings of the SIGCHI Conference on Human Factors in Computing Systems},
}
\end{filecontents}

\begin{document}

Here's a ligature: \ae

\nocite{*}
\bibliographystyle{alpha}
\bibliography{\jobname}
\end{document}

This doesn't work - it gives this error:

! Undefined control sequence.
l.5 Hornb\aek

but if the \ae are removed from the citation, the correct ligature is displayed in the body of the document (so the \ae is right, just not in the citation). I attempted wrapping the command in {..}:

author = {... Hornb{\ae}k, Kasper},

but this gave me the same error. If I remove \ae from the citation, though, I don't get the ligature inserted for me.

How can I use an explicit ligature in a bibliographic reference?