cannot find or create new `voice'

[cgodefro free]

Hello All,

i'm wondering why it can get this warning by time to time, and how to get rid of it :

# -*-compilation-*-
Processing `C:/Users/cgodefro/Music/Enesco/op1_6.ly'
Parsing...
Interpreting music...
warning: cannot find or create new `voice'
Preprocessing graphical objects...
Finding the ideal number of pages...
Fitting music on 1 page...
Drawing systems...
Layout output to `/Users/cgodefro/Music/Enesco/op1_6.ps'...
Converting to `/Users/cgodefro/Music/Enesco/op1_6.pdf'...
success: Compilation successfully completed

the source file:

\version "2.14.0"
\include "italiano.ly"
\header { }
#(set-global-staff-size 15)
TimeKey = { \key do \minor \time 12/8}


TOne= \relative do'' {
            \clef treble
            \TimeKey
            sol'8 fa mib re4.~\(re4 \<mib8\! \) re4 (\<mib8\!)
            sol (fa mib) re4.~\(re4 \<mib8\! \) re4 (\<mib8\!)
            sib' (lab sol fa4.) lab8 (sol) mib (sol fa) re (fa mib re do) r r r2.
            }

\score {
    \new Staff {
    \set Staff.instrumentName = \markup{ \fontsize #2 {"Violons I"} \hspace #1.0 }
    \new voice = "op1_6" {\TOne}
    }
\layout {}
}


in fact this message polute my logs from latex and i'd like first to understand why it appears and how to resolve it. 

Thanks all for your help.

Christophe