[NTG-context] \phantom{\item ... }

Using \phantom to hide an itemized item does not reserve the
(vertical) space for the item as I expect.  That is, in the following
example

\starttext
\startitemize
\item Foo
\phantom{\item Bar}
\item Boo
\stopitemize
\stoptext

the second item is hidden, but in fact, the space for that item has
not been allocated at all.  Why is this, and more importantly to me,
how can I get the space allocated without using hackish transparent
colors?