highlight= with asciidoctor-pdf

I'm trying to get line level (change) highlighting to work with asciidoctor-pdf.  My .asc file has

:source-highlighter: pygments
...

[source,python,highlight=4]
-----
include::foo.py[tags=bar]
-----

When I generate the pdf it syntax-highlights the code snippet as python but does not highlight the 4th line of the included segment in any unique way.  I'm expecting maybe a darker background for that line.

What am I missing?
Thank you.

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