Fretboard diagrams: Why does LilyPond sometimes try to interpret ":7" as a duration?

[Kevin Cole]

Hi,

I decided to move all of the fretboards from various scores to a
"summary" score at the top -- sort of a "legend" so that the scores
would not take up so much space.

I'm defining a variable named frets and then, later:

%%%%
\score {
  <<
    \new ChordNames \frets
    \new FretBoards \frets
  >>
  \layout { }
}

\score {
  <<
    \new ChordNames \chordNames
    \new Voice = "mel" { \melody }
    \new Lyrics \lyricsto mel \words
  >>
  \layout { \autoBeamOff }
}
%%%%

Two files:

> %%%%
> frets = \chordmode {  f4 c4:7 g4:7 c4 bf4 d4:m }
> %%%%

and

> %%%%
> frets = { f4 bf4 c4 }
> %%%%

respectively, work fine. The third has a bit more going on, but adding:

> %%%%
> frets = { g4 c4 a4 d4:7 }
> %%%%

now gives me:

> error: not a duration
>  g4 c4 a4 d4:
>              7

(Without the frets score, everything is fine.)