macro syntax-error works in prefix but not curly-infix

[Damien Mattei <damien.mattei@gmail.com>]

hi,

i have this macro:

(define-syntax <+
  (syntax-rules ()
    ((_ var expr) (define var expr))
    ((_ err ...) (syntax-error "Bad <- form")) ;; does not work in infix !
    ))

why my syntax-error pattern never reach in infix:

scheme@(guile-user)> {x <+ 7 8}
While compiling expression:
Syntax error:
unknown file:3:3: source expression failed to match any pattern in form <+

but ok in prefix:

scheme@(guile-user)> (<+ x 9 10)
While compiling expression:
Syntax error:
unknown location: <+: Bad <- form in form (<+ x 9 10)

why?
Regards,
Damien