diff --git a/racket/src/io/port/string-port.rkt b/racket/src/io/port/string-port.rkt
index 9512f94a2e..1114911a31 100644
--- a/racket/src/io/port/string-port.rkt
+++ b/racket/src/io/port/string-port.rkt
@@ -19,4 +19,4 @@
   (check who (lambda (v) (and (output-port? o) (string-port? o)))
          #:contract "(and/c output-port? string-port?)"
          o)
-  (bytes->string/utf-8 (get-output-bytes o) #\?))
+  (bytes->string/utf-8 (get-output-bytes o) #\uFFFD))