Staff.keySignature misbehaving?

[Graham King]

I'm currently setting a piece by Robert Carver (c.1485-c1570) in which two of the parts have a B-flat in the key signature that applies only to that particular octave.
For this I'm using Staff.keySignature as in the following example:

\version "2.19.5"

\relative c' {
    \set Staff.keySignature = #`(((-1 . 6) . ,NATURAL)
                                 (6 . ,FLAT))
    c4 b c b
    c b c b
}

Unfortunately, lilypond does not recognise that the B a semitone below middle C in the second and subsequent bars is naturalised by the key signature, and it insists on printing superfluous natural signs.  Am I missing something?  Or is this a bug?