diff --git a/ChangeLog b/ChangeLog
index a4c1bc6d68..419864a262 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sat May 25 08:58:23 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (rb_str_quote_unprintable): check if argument is a string.
+
 Fri May 24 19:32:00 2013  Charlie Somerville  <charliesome@ruby-lang.org>
 
 	* variable.c (set_const_visibility): use rb_frame_this_func() instead
diff --git a/string.c b/string.c
index aef65e74c7..fbd496ea7c 100644
--- a/string.c
+++ b/string.c
@@ -8152,8 +8152,10 @@ rb_str_quote_unprintable(VALUE str)
     rb_encoding *enc;
     const char *ptr;
     long len;
-    rb_encoding *resenc = rb_default_internal_encoding();
+    rb_encoding *resenc;
 
+    Check_Type(str, T_STRING);
+    resenc = rb_default_internal_encoding();
     if (resenc == NULL) resenc = rb_default_external_encoding();
     enc = STR_ENC_GET(str);
     ptr = RSTRING_PTR(str);