diff --git a/ChangeLog b/ChangeLog
index 3ef5b9d21f..06894dd4fe 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sat Jan 14 12:02:55 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* sprintf.c (rb_enc_vsprintf): relaxed the restriction.  since the
+	  implementation deeply depends on plain char, so wchar_t based
+	  encodings are not supported.
+
 Sat Jan 14 12:00:20 2012  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* error.c (exc_equal): ignore exceptions during implicit
diff --git a/sprintf.c b/sprintf.c
index bae6c146ac..c34d906310 100644
--- a/sprintf.c
+++ b/sprintf.c
@@ -1169,8 +1169,9 @@ rb_enc_vsprintf(rb_encoding *enc, const char *fmt, va_list ap)
     f._w = 120;
     result = rb_str_buf_new(f._w);
     if (enc) {
-	if (!rb_enc_asciicompat(enc)) {
-	    rb_raise(rb_eArgError, "cannot construct ASCII-incompatible encoding string: %s",
+	if (rb_enc_mbminlen(enc) > 1) {
+	    /* the implementation deeply depends on plain char */
+	    rb_raise(rb_eArgError, "cannot construct wchar_t based encoding string: %s",
 		     rb_enc_name(enc));
 	}
 	rb_enc_associate(result, enc);