[plt-scheme] Illegal Use of #%plain-module-begin

As an exercise, I tried to write a syntax transformation which wraps all
constants in "begin" expressions which print them. This was my first
attempt:

   (define (annotate1 stx)
    (syntax-case stx (#%datum)
      [(#%datum . d) #'(begin (#%app write (#%datum . d)) (#%datum . d))]
      [(first . rest) #`(#,(annotate1 #'first) . #,(annotate1 #'rest))]
      [else #'else]))

The result, printed as an s-exp using syntax-object->datum, looked 
promising, but evaluation of the result produces the error 
"#%plain-module-begin: illegal use (not a module body)". Manually 
expanding the result, i.e. (expand (annotate1 src)), also produces this 
error.

This second transformation, however, produces syntax which evaluates 
cleanly:

   (define (annotate2 stx)
    (syntax-case stx (module #%plain-module-begin #%datum)
      [(#%datum . d) 
       #'(begin (#%app write (#%datum . d)) (#%datum . d))]
      [(module name lang (#%plain-module-begin . bodies))
       #`(module name lang (#%plain-module-begin . #,(annotate2 #'bodies)))]
      [(first . rest)
       #`(#,(annotate2 #'first) . #,(annotate2 #'rest))]
      [else #'else]))

There appears to be some relationship between the #%plain-module-begin
and the enclosing module which only annotate2 preserves, but I'm having
trouble identifying it (or maybe there's not).

Can someone explain this?

Thanks,
Casey