Unable to produce slur in ossia staff

[Seng Liang]

Hi, my name is Seng and I seem to have run into a problem when using 
Lilypond 2.18.2 on Windows 7. I'm trying to include an ossia staff which 
contains a principal note and two ending grace (Aftergrace) notes. I also 
want a slur to connect the principal note and the second grace note, but 
when I compiled the score, Lilypond gives a warning where it tells me it 
cannot end the slur and there is an unterminated slur. The output file 
does not have any slur at all. Here is an example of the problem:

\version "2.18.2"
\new Staff = "main" \relative c'' { \time 2/4
  c2
  <<
    { c2  }

    \new Staff \with {
      \remove "Time_signature_engraver"
      alignAboveContext = #"main"
      firstClef = ##f 
    }
    { \afterGrace c2( { b16[ c]) }
 } >>
  }

What is wrong?

Thank you.
Seng Liang.