NaN encountered, produces small graphical imperfection

[Graham Percival]

The code below produces a whole bunch of errors of this kind:
programming error: Inf or NaN encountered
continuing, cross fingers
programming error: Insane offset
continuing, cross fingers

That's not so serious, but there's also a mistake in the graphical 
output.  In the top-left corner of an invisible box around the first 
g1, there's a dot.  When I zoom in (with Preview), it looks like the 
bottom half of a circle.

I don't necessarily consider this critical or serious; that's up to you 
guys.  :)

% 2.7.38
\relative c <<
  \clef bass
  { <c g>1 ~ <c g>4 } \\
  { e,4 d e2 ~ e4}
>>