diff --git a/collects/tests/mzscheme/unicode.ss b/collects/tests/mzscheme/unicode.ss
index 631801280b..c811f45bcf 100644
--- a/collects/tests/mzscheme/unicode.ss
+++ b/collects/tests/mzscheme/unicode.ss
@@ -1036,6 +1036,8 @@
 (test '(#o302 #o251) bytes->list (unicode-vector->bytes (vector 169)))
 (test '(#o304 #o250) bytes->list (unicode-vector->bytes (vector 296)))
 
+(test "a\0b" bytes->string/utf-8 #"a\xFFb" (integer->char 0))
+
 (test '("\uA9") regexp-match #rx"." "\uA9")
 (test '(#"\302") regexp-match #rx#"." #"\302\251")
 (test '(#"\302") regexp-match #rx#"." "\uA9")
diff --git a/src/mzscheme/src/string.c b/src/mzscheme/src/string.c
index 5b98d21cf4..194ecc9129 100644
--- a/src/mzscheme/src/string.c
+++ b/src/mzscheme/src/string.c
@@ -1104,7 +1104,7 @@ do_byte_string_to_char_string(const char *who,
 		NULL, 0, 
 		(perm > -1) ? 0xD800 : 0);
   
-  if (perm > 0) {
+  if (perm > -1) {
     for (i = 0; i < ulen; i++) {
       if (v[i] == 0xD800)
 	v[i] = perm;