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))))