> --- a/lisp/char-fold.el
> +++ b/lisp/char-fold.el
> @@ -28,7 +28,6 @@
>    (defun char-fold-make-table ()
>      (let* ((equiv (make-char-table 'char-fold-table))
>             (equiv-multi (make-char-table 'char-fold-table))
> -           (search-spaces-regexp nil)   ; workaround for bug#35802

Ah, I guess this part explains why search-spaces-regexp turned up nil in
the debugger backtrace.  So I think adjusting isearch-search-fun-default
should be enough to fix this.