diff --git a/LOG b/LOG
index fc9f64c523..28919e742b 100644
--- a/LOG
+++ b/LOG
@@ -763,4 +763,6 @@
   to use define-who instead of repeating 'substring-fill! in all the error
   messages.
     5_4.ss, 5_6.ss
-
+- fix mat of substring-fill!
+  after the recent change, the result of substring-fill! is void
+    5_5.ms
diff --git a/mats/5_5.ms b/mats/5_5.ms
index 53c367b828..f1a1e5eac9 100644
--- a/mats/5_5.ms
+++ b/mats/5_5.ms
@@ -699,8 +699,12 @@
     (error? (substring-fill! 'a 0 1 #\a))
     (error? (substring-fill! "hi" 0 'a #\a))
     (error? (substring-fill! "hi" 1 0 #\a))
-    (equal? (substring-fill! "hitme!" 0 5 #\a) "aaaaa!")
-    (eqv? (substring-fill! "" 0 0 #\a) "")
+    (let ([s (string-copy "hitme!")])
+      (substring-fill! s 0 5 #\a)
+      (equal? s "aaaaa!"))
+    (let ([s ""])
+      (substring-fill! s 0 0 #\a)
+      (eqv? s ""))
     (let ([s (string-copy "ABCDE")])
        (and (begin
                (substring-fill! s 0 0 #\$)