[NTG-context] "Invalid field" in Mark IV

[Flavien Lambert]

Dear all, I didn't use ConTeXt for some time and, so, have just re-installed
the minimals tonight. When running on some of my files, Mark II does its job
well but Mark IV tells me

error: Invalid field id nucleus for node type choice (0)
.
l.133 ... M décrit l'axe $({\rm O};\vect{e_{x}})$
                                                   avec une vitesse
$\dot{x}...

I have absolutely no idea what it is trying to say to me...
If related, the macro is defined by
\def\vect#1{%
  \overrightarrow{#1}
}

Regards,
-- 
Flavien.
----------------------------
"Quantum objects are completely crazy but, at least, they are all crazy in
the same way." R.P. Feynman.
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