Strange harp problem

[Haipeng Hu]

Hello,
  I'm composing a piece. The attached harp part is ok until the last
two lines of each part. When I comment out them, there is no error.
When I leave them to be compiled, the following error appears, and I
can't figure out what's wrong at all:
programming error: no note heads for the line spanner on neighbor
line? Confused.
continuing, cross fingers
What's wrong on earth?

Regards
Haipeng

harp.ly
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