Music for the Martians?

[arjan.bos]

Dear List,

When upgrading from 2.8.6 to 2.10.0 one of my files was producing this strange error message:

Preprocessing graphical objects...
/Users/arjanbos/Music/Gitaarles/Boek-2/MusicForTheMartians.ly:12:28: Music for the martians
      e8_1 e_2 e_3 f_4 f_5 f
                            _6 |

What is music for the martians? The purpose of the file is to help beginning classical guitar players to understand a 6/8 beat. The numbers are there to indicate the counting. The solution I found is to simply put the f_6 as f_"6".
But what are martians and why are they playing my music? Shouldn't this message simply state that a 6 is a strange number to indicate a finger with? But I know that already, and obviously I want the 6 to be in, otherwise it wouldn't be in. So clearly it is not meant to indicate a finger, so I feel that the message can safely be removed from Lilypond. ;-)

The smallest file I found to reproduce it is
\version "2.10.0"
\score {
  \context Staff = guitar {
    \key c \major \time 6/8
    \relative c'' {
      e8_1 e_2 e_3 f_4 f_5 f_6 |
    }
  }
}


---
Look, that's why there's rules, understand? So that you think before you break 'em.

-- (Terry Pratchett, Thief of Time)