guitar music: do I have to treat it as 2 voices?

[arie-lily]

Hello all, I have a piece of guitar music, treating it as one voice.
Lily 1.6.10 prints what I want, but gives the following warning:

warning: Adding note head
to incompatible stem (type = 4):
        e2.-\p ~ < e
                    2 {r4 r4 r8} > b'8-\p

where my music is as follows:

\score { 
    \notes {
        \key e \major
        \time 6/8
        e2.-\p ~ < e2 {r4 r4 r8} > b'8-\p
    }
}

Is there a way to prevent this, or am I 'obliged' to treat this guitar
music as two voices, i.e. bass and treble, and put those two voices on
one staff?

thanks,
arie