diff --git a/contrib/20.r7rs/src/mutable-string.c b/contrib/20.r7rs/src/mutable-string.c
index 85db9be0..01887976 100644
--- a/contrib/20.r7rs/src/mutable-string.c
+++ b/contrib/20.r7rs/src/mutable-string.c
@@ -4,13 +4,16 @@ void
 pic_str_set(pic_state *pic, pic_str *str, size_t i, char c)
 {
   pic_str *x, *y, *z, *tmp;
+  char buf[1];
 
   if (pic_str_len(str) <= i) {
     pic_errorf(pic, "index out of range %d", i);
   }
 
+  buf[0] = c;
+
   x = pic_str_sub(pic, str, 0, i);
-  y = pic_make_str_fill(pic, 1, c);
+  y = pic_make_str(pic, buf, 1);
   z = pic_str_sub(pic, str, i + 1, pic_str_len(str));
 
   tmp = pic_str_cat(pic, x, pic_str_cat(pic, y, z));
diff --git a/extlib/benz/include/picrin/string.h b/extlib/benz/include/picrin/string.h
index 2728e97b..b7448783 100644
--- a/extlib/benz/include/picrin/string.h
+++ b/extlib/benz/include/picrin/string.h
@@ -22,7 +22,6 @@ void pic_rope_decref(pic_state *, struct pic_rope *);
 
 pic_str *pic_make_str(pic_state *, const char * /* nullable */, size_t);
 pic_str *pic_make_str_cstr(pic_state *, const char *);
-pic_str *pic_make_str_fill(pic_state *, size_t, char);
 
 char pic_str_ref(pic_state *, pic_str *, size_t);
 size_t pic_str_len(pic_str *);
diff --git a/extlib/benz/string.c b/extlib/benz/string.c
index 9d1060c3..cc4a0519 100644
--- a/extlib/benz/string.c
+++ b/extlib/benz/string.c
@@ -240,25 +240,6 @@ pic_make_str_cstr(pic_state *pic, const char *cstr)
   return pic_make_str(pic, cstr, strlen(cstr));
 }
 
-pic_str *
-pic_make_str_fill(pic_state *pic, size_t len, char fill)
-{
-  size_t i;
-  char *buf = pic_malloc(pic, len);
-  pic_str *str;
-
-  for (i = 0; i < len; ++i) {
-    buf[i] = fill;
-  }
-  buf[i] = '\0';
-
-  str = pic_make_str(pic, buf, len);
-
-  pic_free(pic, buf);
-
-  return str;
-}
-
 size_t
 pic_str_len(pic_str *str)
 {
@@ -474,7 +455,16 @@ pic_str_make_string(pic_state *pic)
 
   pic_get_args(pic, "k|c", &len, &c);
 
-  return pic_obj_value(pic_make_str_fill(pic, len, c));
+  {
+    size_t i;
+    char buf[len];
+
+    for (i = 0; i < len; ++i) {
+      buf[i] = c;
+    }
+
+    return pic_obj_value(pic_make_str(pic, buf, len));
+  }
 }
 
 static pic_value