`generate-term` + missnamed cross gives an internal error message

[florence]

the program

#lang racket
(require redex/reduction-semantics)

(define-language L
  (b ::= a))

((generate-term L (cross b_1))
 1)

Gives the error

redex/private/rg.rkt:182:2: hash-ref: no value found for key
  key: 'b_1-b_1

Instead of complaining that b_1 isn't a non-terminal in L.

Also if I add a #:i-th:

#lang racket
(require redex/reduction-semantics)

(define-language L
  (b ::= a))

((generate-term L (cross b_1) #:i-th)
 1)

The error message points to the middle of the contract library:

collects/racket/contract/private/arrow-val-first.rkt:390:18: hash-ref: no value found for key
  key: 'b_1-b_1