Manually override code names?

[Tom Campbell]

Using a rootless version of C13 in the piano left hand makes Lilypond call it an E variant. I can't figure out how to override chord names so I'm using markup, like this. What am I doing wrong?

leftHand = \relative c, {
  \clef bass
  \key c \major
  \time 4/4

  <e' a bes d>4^\markup { C13 }
  <e a bes d>4
  <ees g a  d>^\markup { F13 }
  <ees g a  d>
}

 <<
    \new Staff = "lower" \leftHand
 >>
}

Thanks!