strange error

[Michael Vanier]

This file:

% Begin lilypond file.

melodya = \notes \relative c'' {
        \stemUp
        d4 c4 b4 a4
}


melodyb = \notes \relative c' {
        \stemDown
        d2 b2
}


\score {
        \simultaneous {
            \context Staff = melodya <
                \time 4/4;
                \clef treble;
                \melodya
                \melodyb
            >
        }

        \paper {
            linewidth = 15.0\cm;
        }
}

% End lilypond file.


generates errors as follows:

test.ly:13:12: warning: Adding note head to incompatible stem (type = 4):
        d2 b
            2

test.ly:14:1: warning: Adding note head to incompatible stem (type = 4):
}

Lilypond seems to ignore the \stemUp directive.  This is using 1.3.120.  Is
there any way to force an upward-pointing stem?

Thanks,

Mike