Genius. I changed the papersize parameter variable value from 'a4' to 'A4' and the problem disappeared. Thanks for the help -- that would have taken me forever to track down.

On Monday, 7 August 2017 14:30:56 UTC+9:30, John MacFarlane wrote:

Can you give exact instructions (including any inputs) for
recreating the problem, so we can test?

Note: you might try setting papersize to 'A4' (capital
letter A), instead of 'a4'.  I doubt that's the issue
here, but it is one thing we've fixed recently (pandoc
dev version will automatically capitalize 'a4' for
ConTeXt output).

+++ David Nebauer [Aug 06 17 19:55 ]:
>   When creating pdf output using context, pandoc fails with the error
>   message:
>   tex error on line 15 in file /tmp/tex2pdf.2478/input.tex: ! Missing
>   number, treated as zero
>   On creating context output line 15 turns out to be:
>   \definefontfeature[default][default][script=latn,
>   protrusion=quality, expansion=quality, itlc=yes, textitalics=yes,
>   onum=yes, pnum=yes]
>   On processing the context output directly I get the following expanded
>   error message for line 15:
>   Dimensions can be in units of em, ex, in, pt, pc,
>   cm, mm, dd, cc, nd, nc, bp, or sp; but yours is a new one!
>   I'll assume that you meant to say pt, for printer's points.
>   To recover gracefully from this error, it's best to
>   delete the erroneous units; e.g., type `2' to delete
>   two letters. (See Chapter 27 of The TeXbook.)
>   tex error       > tex error on line 15 in file FILEPATH.tex: ! Illegal
>   unit of measure (pt inserted)
>   In case it helps, here is the context output up to and including line
>   15:
>   % Enable hyperlinks
>   \setupinteraction
>     [state=start,
>     title={Minimal Working Example},
>     style=,
>     color=blue,
>     contrastcolor=]
>   % make chapter, section bookmarks visible when opening document
>   \placebookmarks[chapter, section, subsection, subsubsection,
>   subsubsubsection, subsubsubsubsection][chapter, section]
>   \setupinteractionscreen[option=bookmark]
>   \setuptagging[state=start]
>   \setuppapersize[a4]
>   % use microtypography
>   \definefontfeature[default][default][script=latn, protrusion=quality,
>   expansion=quality, itlc=yes, textitalics=yes, onum=yes, pnum=yes]
>   Pandoc version is 1.17.2 on debian/testing.
>
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