nanopass-case producing unexpected error

[akeep]

(originally submitted over email to the nanopass-framework mailing list by Axel Søgaard)

The following programs compiles without errors:

lang nanopass

(define variable? symbol?)

(define-language LFE (entry TopLevelForm) (terminals (variable (x xd)))
(Formals (f) x)
(TopLevelForm (t) g)
(GeneralTopLevelForm (g) e) (Expr (e) x))

(define-language LFE2 (extends LFE) (Abstraction (ab) (+ (λ f e) => (λ f e))) (Expr (e) (+ ab)))

(define-language LANF (extends LFE2) (AExpr (ae) (+ x ab)) (CExpr (ce) (+ ae)) (Expr (e) (+ ae ce)))

However, if I add the following expression, I get an error:

(λ() (nanopass-case (LANF Expr) 43 [else 4]))

The (internal) compile time error I get is:

Dropbox/GitHub/nanopass-framework-racket/private/pass.rkt:1059:52: memf: not a proper list: '#s((nonterminal-alt #(0) alt 3) #<syntax:27:7 ae> #f #f #<syntax:21:3 AExpr>)

I know that 43 isn't an LANF Expression, but that's besides the point.

Note that if I change

(Expr (e) (+ ae ce)))

to

(Expr (e) (+ ce)))

the error disappears.

Is the language definition correct?