Warning about beam slope configuration

[David Sumbler]

%%%%%%%%%%%%%%%%
\version "2.19.48"

global = { \key e \minor \time 2/4 }

\score {
  <<
    \new StaffGroup <<
      \new Staff {
        \global r4 b''16( ais'' b'' b''') 
      }
      \new Staff {
        \clef "tenor" \global
        b16( ais b e') dis'( b fis) r
      }
    >>
    \new Staff {
      { \clef "bass" \global b8 e' b4 }
      \addlyrics { \lyricmode { la, la, la. } }
    }
    \new PianoStaff <<
      \new Staff = "RH" {
        { \global b'16 ais' g' b r4 }
      }
      \new Staff = "LH" {
        \clef "bass" \global
        r4 b,16 fis b \change Staff = RH dis' \change Staff = LH
      }
    >>
  >>
}
%%%%%%%%%%%%%%%%

The above compiles satisfactorily.

However, for some unknown reason, that bar throws out a warning when
the whole file from which it is taken is compiled.  I get:

warning: no viable initial configuration found: may not find good beam
slope
  r4 
     b,16 fis b \change Staff = RH dis' \change Staff = LH

Also the beam of the offending group of notes is printed above the
lower-staff notes and below the upper-staff note, rather than below all
4 notes as it is in the above extract with the bar taken in isolation.

Presumably something that has gone earlier in the score has somehow
upset things, although I cannot imagine what it could be.

Before I start taking the file apart to find out what earlier feature
is causing the problem, could somebody explain to me what the warning
actually means, and what I would need to do to stop it appearing?

David