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.

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