Internal error from make-language

[soegaard]

The following program gives the error:

../../Applications/Racket v6.2/collects/racket/contract/private/blame.rkt:143:0: make-language: contract violation expected: identifier? given: #f in: the 2nd argument of (-> identifier? identifier? (listof tspec?) (listof ntspec?) language?) contract from: /nanopass-framework-racket/private/records.rkt blaming: /nanopass-framework-racket/private/language.rkt (assuming the contract is correct) at: /nanopass-framework-racket/private/records.rkt:65.5

#lang nanopass
(define variable? symbol?)

(define-language L
  (entry Expr)
  (terminals
   (variable (x)))
  (Expr (e)
    x))

(define-language L1
  (terminals
   (+ (variable l))))