diff --git a/ChangeLog b/ChangeLog
index 7965d343cd..ae38b28900 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Dec 18 01:46:48 2007  Yukihiro Matsumoto  <matz@ruby-lang.org>
+
+	* sprintf.c (rb_str_format): need to clear output buffer to avoid
+	  broken encoding compatibility check.
+
 Tue Dec 18 01:40:20 2007  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* lib/mkmf.rb (depend_rules): inserts ruby to only headers.
diff --git a/sprintf.c b/sprintf.c
index f005ad6af4..b59e3d1401 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -298,6 +298,7 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
     result = rb_str_buf_new(bsiz);
     rb_enc_copy(result, fmt);
     buf = RSTRING_PTR(result);
+    memset(buf, 0, bsiz);
 
     for (; p < end; p++) {
 	const char *t;
@@ -850,7 +851,6 @@ rb_str_format(int argc, const VALUE *argv, VALUE fmt)
     rb_str_resize(result, blen);
 
     if (tainted) OBJ_TAINT(result);
-    rb_enc_associate(result, enc);
     return result;
 }