can't customize compare-windows to have previous behaviour

[Chris Moore]

The documenation string for compare-windows tells me:
  If `compare-windows-sync' is non-nil, then successive calls of
  this command work in interlaced mode

Also, the documentation string for compare-windows-sync tells me:
  If the value of this variable is `nil', then function `ding' is
  called to beep or flash the screen when points are mismatched.

Yet when I go to customise compare-windows-sync, I don't see any
option to set it to nil; all I seem to be able to chose is to set it
to a function or a regexp.

I tried setting it to function nil, but was told:
  custom-variable-save: Saving compare-windows-sync: Invalid function: nil

I also tried setting it to regexp nil, but then I see:
  compare-windows-sync is a variable defined in `compare-w.el'.
  Its value is "nil"

ie. it's set to the string "nil", not to nil itself.

So how can I get back the old behaviour of compare windows, where it
stops at differences, rather than skipping over them in some
semi-clever way?

In GNU Emacs 22.0.96.5 (i686-pc-linux-gnu, GTK+ Version 2.8.20)
 of 2007-03-22 on trpaslik
Windowing system distributor `The X.Org Foundation', version 11.0.70101000
configured using `configure  '--with-gtk' '--prefix' '/usr/local' '--with-xpm' 
'--with-jpeg' '--with-png' '--with-gif''