internal error via judgment form

Racket Version

6.12.0.3--2018-02-07(-/f) [3m].

Program

(define-language Flow
  (p ::= ((node x e) ... (solve x x e)))
  (e ::= x integer (edge x e))
  (x ::= (variable-except node edge solve))
  #:binding-forms
  ((node x_n e_n) #:refers-to (shadow x_n ...) ...
   (solve x_from x_to e #:refers-to (shadow x_n ...))))

(define-judgment-form Flow
  #:mode (⊢p I)
  [------------------------------------------------- p
   (⊢p ((node x_n e_n) ... (solve x_from x_to e)))])

(judgment-holds (⊢p ((node b (edge d 30)) (solve a d argmax))))

Expected

a Boolean

Got

  expected: list?
  given: 'b«0»

Hints

1. Removing integer makes the problem disappear.
2. (judgment-holds (⊢p ((node b (edge d 30)) (node d) (solve a d argmax)))) also makes the problem disappear.

The above is distilled down from about 5 pages. I tried to make it smaller and couldn't.

Start by reproducing the judgment-holds example with define-language, define-judgment-form, and the integer production, then compare it with the two hint variants. Trace the source of the second: contract violation and determine why the judgment form returns 'b«0» instead of a Boolean. Done means the original example returns a Boolean without regressing the hint cases.