[racket] define-for-syntax and racket/base

Why does the following module give an error, "compile: unbound 
identifier in the transformer environment (and no #%app syntax 
transformer is bound) in: string-append", but does not give an error if 
"racket/base" is changed to "racket"?

#lang racket/base
(define-for-syntax aaa (string-append "x" "y" "z"))

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