Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2020262 
(FPCore (x)
  :name "sqrtexp (problem 3.4.4)"
  :precision binary64
  (sqrt (/ (- (exp (* 2.0 x)) 1.0) (- (exp x) 1.0))))

Backtrace

hash-ref: no value found for key key: #f		L	C
table-ref	/data/pavpan/nightlies/herbie/master/src/common.rkt	32	0
f656	unknown
for-loop	/data/pavpan/nightlies/herbie/master/src/programs.rkt	327	11
f656	unknown
for-loop	/data/pavpan/nightlies/herbie/master/src/programs.rkt	327	11
f656	unknown
for-loop	/data/pavpan/nightlies/herbie/master/src/programs.rkt	327	11
f656	unknown
f689	unknown
f689	unknown
expand-parametric	/data/pavpan/nightlies/herbie/master/src/programs.rkt	289	0
(unnamed)	/opt/racket-7.8/collects/racket/contract/private/arrow-higher-order.rkt	375	33
loop	/data/pavpan/nightlies/herbie/master/src/programs.rkt	98	4
loop	/data/pavpan/nightlies/herbie/master/src/programs.rkt	98	4
(unnamed)	/opt/racket-7.8/collects/racket/contract/private/arrow-higher-order.rkt	375	33
for-loop	/data/pavpan/nightlies/herbie/master/src/mainloop.rkt	151	6
taylor-alt	/data/pavpan/nightlies/herbie/master/src/mainloop.rkt	144	0
gen-series!	/data/pavpan/nightlies/herbie/master/src/mainloop.rkt	166	0
run-iter!	/data/pavpan/nightlies/herbie/master/src/mainloop.rkt	360	0
for-loop	/data/pavpan/nightlies/herbie/master/src/mainloop.rkt	391	2
run-improve	/data/pavpan/nightlies/herbie/master/src/mainloop.rkt	377	0
(unnamed)	/data/pavpan/nightlies/herbie/master/src/sandbox.rkt	70	6
run	/opt/racket-7.8/share/pkgs/profile-lib/main.rkt	39	2
profile-thunk	/opt/racket-7.8/share/pkgs/profile-lib/main.rkt	9	0
(unnamed)	/opt/racket-7.8/collects/racket/engine.rkt	42	24