MkIV fails with

    tex error       > tex error on line 3 in file c://Users/micro/Desktop/fail.tex: ! Missing \endcsname inserted

given the following mwe:

    \starttext
    a
    \blank[\lineheight]
    z
    \stoptext

Not critical as there are lots of ways to do this, but it seems strange 
that it should work in one and not the other.

-- 
Rik