Error message using AikenHeads with NullVoice

[David F.]

I have a collection of SATB hymns that I would like to engrave using shape 
notes.  The Lilypond files are all structured the same, as seen below—treble 
clef with music and a NullVoice for aligning lyrics, then the lyrics, then the 
bass clef.

%%%%%
\version "2.22"
\language “english"

% included from another file
\layout {
    \context {
        \Staff
            \aikenHeads
    }
}

\score {
    <<
        \new Staff <<
            % music here
            \new NullVoice = "melody" { e'8 fs'8 }
        >>
        % \new Lyrics \lyricsto "melody" { Ly -- rics }
        % bass clef here
    >>
    \layout {}
}
%%%%%

The command to use aikenHeads comes from a separate style file that gets 
included for each hymn.  But there is an unfortunate interaction between 
aikenHeads and NullVoice: Lilypond outputs the following error message:

programming error: must have stem dir for note head
continuing, cross fingers

And it outputs a lot of these messages.  I need to find a way to stop Lilypond 
from outputting this error.  Any kind of quick fix or hack would be sufficient.

One thought I had would be to re-enable regular note heads for the NullVoice:

            \new NullVoice = "melody" \with { \unset shapeNoteStyles } { e'8 
fs'8 }

But I couldn’t come up with anything that worked.

Any help or ideas would be greatly appreciated.

David F.