syntax-rules problem

I have some trouble to understand what's wrong with the following macro:

  (begin
    (define-syntax define-a
      (syntax-rules ()
        ((_) (define a 0))))
    (define-a)
    a)

I would expect that after evaluating (define-a), a is defined
and that the entire form would return 0. 

However, Scheme48 (version 1.8) signals an undefined variable error.
Is this a bug or a feature?

Helmut.