I render a multipart book with Asciidoctor PDF.

In a page’s header, I want to have the current book part and chapter, separated by –. If there is no book part (e.g. in preface or appendix) there should only be the chapter title.

I tried to use a conditional expression in my theme:

extends: default
header:
  height: 33pt
  recto: &header
    content-margin: [8pt, inherit, inherit, inherit]
    right:
      content: ifdef::part-title[{part-title} – ] {chapter-title}
  verso: *header

However, this just renders ifdef::part-title[ – ] as-is, without evaluating the conditional expression (while it does evaluate {part-title}).

How can I fix this to actually evaluate the conditional expression?