error in bar number snippet?

[Andrew Hawryluk]

In the snippet printing-the-bar-number-for-the-first-measure.ly (NR
1.2.5), there appears to be an error.
It recommends

\set Score.barNumberVisibility = #all-bar-numbers-visible
\bar ""

but the first line gives a warning: type check for
`barNumberVisibility' failed; value `#<unspecified>' must be of type
`procedure'

It turns out that I can get a bar number on the first line just from
the empty bar line, no \set required.

Andrew