strange warnings

[Michael Vanier]

I'm getting strange warning messages in a lilypond file that are unrelated
to the contents of the line being warned about:

foo.ly:147:6: warning: unbound spanner `Beam':
    s1
      
foo.ly:147:6: warning: beam has less than two visible stems:
    s1

foo.ly:147:6: warning: Beam has less than two stems. Removing beam.:
    s1
      
Line 147 just has "s1" i.e. there's nothing there.  Originally there was
something there, but whatever is there gives exactly the same warning
messages.  Since the warnings are unrelated to the contents of the line,
how do I tell what's wrong?

Mike