Custos style mensural

[Nicolas Sceaux]

Using lily 2.1.25 on Debian unstable.

In this example taken from the doc, the \break command does not work
as expected:

\score {
    \notes {
        a'1
        \override Staff.Custos   #'style = #'mensural
        \break
        g'
    }
    \paper {
        \translator {
            \StaffContext
            \consists Custos_engraver
        }
        linewidth = 4.0\cm
    }
}

the warning is: 
Could not find line breaking that satisfies constraints.
As a result, there is no line break between the two notes, and thus no
custode.

See:
http://lilypond.org/doc/v2.1/Documentation/user/out-www/lilypond/Custodes.html#Custodes

However, this one is OK:

\score {
    \notes {
        a'1
        \override Staff.Custos   #'style = #'hufnagel
        \break
        g'
    }
    \paper {
        \translator {
            \StaffContext
            \consists Custos_engraver
        }
        linewidth = 4.0\cm
    }
}