adding a tie from a trill'd note to the next note

[Chip]

<<{a2\startTrillSpan}{s4 s8 s16 s32 s32\stopTrillSpan}>> ~ a8 gs a b
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The above code does not work. I move the tilde anywhere in the line and 
it always results in this error:
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19.ly:55:59 error: syntax error, unexpected '~':
<<{a2\startTrillSpan}{s4 s8 s16 s32 s32\stopTrillSpan}>>
                                                ~ a8 gs a b
error: found error in this music expression. Ignoring it.
error: failed file 19.ly
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How do I tie the trilled a2 to the a8?

Thanks,
Chip