latest version breaks some functions

[michael webster]

Hey all - I upgraded from 2.14 to 2.18 and this function which had used to work 
fine now throws an error

metMod = #(define-music-function (parser location valueI valueII) (string? 
string?)
    #{ 
    \once \override Score.RehearsalMark #'outside-staff-priority = #1000
      \mark \markup {
         \concat {
            \smaller \general-align #Y #DOWN \note #$valueI #1
            " = "
            \smaller \general-align #Y #DOWN \note #$valueII #1      
          } 
      }  
    #}
  )

the error is somthing like  "GUILE expects input type string instead of input 
type <unassigned> " at  #$valueI in line below

            \smaller \general-align #Y #DOWN \note #$valueI #1

any idea what's broken??

thanks in advance - cheers Michael