empty midi file output from guitar tab

[Abraham Raher]

This simple line in guitar tab produces a correct pdf score and a midi file, but
the midi file does not have any notes or durations in it. So it plays but makes no
sound.

Can anyone explain what I'm doing wrong? Sorry, am a naive user. Thanks!

 \version "2.10.10"

    guitarline = {
        a,4\5 c'\2 a\3 e'\1
    }
     
     \score {
        <<
           \new TabStaff = "gtr" {

            \guitarline

            } 
                            >>
        \layout {
           \context { \RemoveEmptyStaffContext }
        }
        \midi { 
                }
     }