On 2013–07–16 Steffen Kram wrote:

> A similar problem occurs, when using references:

It might be a parser issue. ConTeXt uses different parsers for
different commands. Most commands happily accept values without
trailing comma. The commands using a different parser require a
trailing comma. I recall this applied to \definepalet, \usepath and
a few others.

The issue occasionally pops up on the mailing list, since it's
inconsistent and non-intuitive behaviour.

  http://www.ntg.nl/pipermail/ntg-context/2012/067585.html
  https://gist.github.com/phi-gamma/2941284#file-features-wiki-L258

Personally I don't use that syntax any more. It doesn't work
reliably and fails occasionally with particular commands.

Marco