Mark formatting problem

[Menu Jacques]

Hello folks,

Mark « B » in the following is not placed in a box, like the others.

Thanks for your help!

JM

—

\version "2.19.55"

\relative c'' {
       \set Score.markFormatter = #format-mark-box-alphabet
       c1 \mark \default
       c1 \mark "B"
       c1 \mark #8
       c1 \mark \default
       c1 \mark \default
}