Exception for Cubic critical

Math

\[\frac{\left(-b\right) + \sqrt{b \cdot b - \left(3 \cdot a\right) \cdot c}}{3 \cdot a}\]

C

double f(double a, double b, double c) {
        double r8382 = b;
        double r8383 = -r8382;
        double r8384 = r8382 * r8382;
        double r8385 = 3.0;
        double r8386 = a;
        double r8387 = r8385 * r8386;
        double r8388 = c;
        double r8389 = r8387 * r8388;
        double r8390 = r8384 - r8389;
        double r8391 = sqrt(r8390);
        double r8392 = r8383 + r8391;
        double r8393 = r8392 / r8387;
        return r8393;
}

Reproduce

Please include this information when filing a bug report:

herbie shell --seed 2019191 
(FPCore (a b c)
  :name "Cubic critical"
  (/ (+ (- b) (sqrt (- (* b b) (* (* 3.0 a) c)))) (* 3.0 a)))

Backtrace

get-representation: Unknown representation #f		L	C
loop	/data/pavpan/nightlies/herbie/interface2/src/points.rkt	122	4
prepare-points	/data/pavpan/nightlies/herbie/interface2/src/points.rkt	146	0
setup-prog!34	/data/pavpan/nightlies/herbie/interface2/src/mainloop.rkt	67	0
run-improve43	/data/pavpan/nightlies/herbie/interface2/src/mainloop.rkt	339	0
(unnamed)	/opt/racket-7.0/collects/racket/private/more-scheme.rkt	261	28
run	/opt/racket-7.0/share/pkgs/profile-lib/main.rkt	39	2
profile-thunk16	/opt/racket-7.0/share/pkgs/profile-lib/main.rkt	9	0
(unnamed)	/opt/racket-7.0/collects/racket/private/more-scheme.rkt	261	28