Difficulty with a missing tie

[Michael Welsh Duggan]

In the following snippet I run into the problem that the c1 ~ c tie is
not recognized.  Does anyone have any idea how I should do this instead?

\version "2.21.0"
\include "english.ly"

{
  \new PianoStaff \with {
    \consists #Span_stem_engraver
  } 
  <<
    \new Staff = "up" {
      r2. << {a'4 ~ }
             \context Staff = "down" \crossStaff {\stemUp a ~} >>
      << a'1 \context Staff = "down" \crossStaff {a} >>
    }
    \new Staff = "down" {
      \clef bass
      c1 ~ c
    }
  >>
}

-- 
Michael Welsh Duggan
(address@hidden)