minor chord produces syntax error

[vector]

Hi,
 script shown below
the log file is indicating a syntax error, unexpected STRING at the "g:m"
I thought that was ok to indicate a gminor?
In fact i cant get any chord modifiers to work at all.

\version "2.16.1"
\header {
  title = "BluesJam Trk4"
  composer = "me"
}

progChords = \relative c {
  \hideNotes
  bes1 es es bes g:m

}

\score {
  <<
    \new ChordNames {
      \set chordChanges = ##t
      \chordmode { \progChords }
    }
    \new Staff = "guitar"
    <<
      \clef "G"
      \key f \major
      \numericTimeSignature
      \time 4/4
      \new Voice = "prog" \progChords
    >>
  >>
  \header {
    piece = "BluesJaminF pg1 tk4"
  }
  \layout {
    \context {
      \Score
      \remove "Bar_number_engraver"
    }
  }
}