Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2021042 
(FPCore (a b c)
  :name "The quadratic formula (r1)"
  :precision binary64
  (/ (+ (- b) (sqrt (- (* b b) (* (* 4.0 a) c)))) (* 2.0 a)))

Backtrace

cdr: contract violation expected: pair? given: 'x		L	C
for-loop	/data/pavpan/nightlies/herbie/mpmf-cost-multi-input/src/programs.rkt	220	4
(unnamed)	/data/pavpan/nightlies/herbie/mpmf-cost-multi-input/src/programs.rkt	216	2
(unnamed)	/data/pavpan/nightlies/herbie/mpmf-cost-multi-input/src/programs.rkt	150	2
(unnamed)	/data/pavpan/nightlies/herbie/mpmf-cost-multi-input/src/points.rkt	169	4
errors	/data/pavpan/nightlies/herbie/mpmf-cost-multi-input/src/points.rkt	166	0
make-alt-table	/data/pavpan/nightlies/herbie/mpmf-cost-multi-input/src/core/alt-table.rkt	33	0
(unnamed)	/opt/racket-7.9/collects/racket/contract/private/arrow-val-first.rkt	486	18
setup-prog!	/data/pavpan/nightlies/herbie/mpmf-cost-multi-input/src/mainloop.rkt	80	0
for-loop	/data/pavpan/nightlies/herbie/mpmf-cost-multi-input/src/mainloop.rkt	453	4
run-improve	/data/pavpan/nightlies/herbie/mpmf-cost-multi-input/src/mainloop.rkt	447	0
(unnamed)	/data/pavpan/nightlies/herbie/mpmf-cost-multi-input/src/sandbox.rkt	54	6
run	/opt/racket-7.9/share/pkgs/profile-lib/main.rkt	39	2
profile-thunk	/opt/racket-7.9/share/pkgs/profile-lib/main.rkt	9	0
(unnamed)	/opt/racket-7.9/collects/racket/engine.rkt	42	24

Reproduce

herbie shell --seed 2021042 
(FPCore (a b c)
  :name "The quadratic formula (r1)"
  :precision binary64
  (/ (+ (- b) (sqrt (- (* b b) (* (* 4.0 a) c)))) (* 2.0 a)))