diff --git a/racket/src/racket/src/symbol.c b/racket/src/racket/src/symbol.c
index 9068fdd366..41b031e670 100644
--- a/racket/src/racket/src/symbol.c
+++ b/racket/src/racket/src/symbol.c
@@ -611,11 +611,11 @@ const char *scheme_symbol_name_and_size(Scheme_Object *sym, uintptr_t *length, i
     if (ch > 127) {
       /* Decode UTF-8. */
       mzchar buf[2];
-      int ul = 2;
+      int ul = 1;
       while (1) {
         if (scheme_utf8_decode((unsigned char *)s, i, i + ul,
                                buf, 0, 1,
-                               NULL, 0, 0) > 0)
+                               NULL, 0, '?') > 0)
           break;
         ul++;
       }