diff --git a/ChangeLog b/ChangeLog
index dd17548b24..2cf84a8608 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Sat Jan 29 22:16:26 2011  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* array.c (rb_ary_join): [].join.encoding must be US-ASCII.
+	  [ruby-list:47790]
+
 Sat Jan 29 20:22:39 2011  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 
 	* benchmark/driver.rb (BenchmarkDriver#measure): Show command line
diff --git a/array.c b/array.c
index 7624e1ffe5..6420e5b645 100644
--- a/array.c
+++ b/array.c
@@ -1681,7 +1681,7 @@ rb_ary_join(VALUE ary, VALUE sep)
     int untrust = FALSE;
     VALUE val, tmp, result;
 
-    if (RARRAY_LEN(ary) == 0) return rb_str_new(0, 0);
+    if (RARRAY_LEN(ary) == 0) return rb_usascii_str_new(0, 0);
     if (OBJ_TAINTED(ary) || OBJ_TAINTED(sep)) taint = TRUE;
     if (OBJ_UNTRUSTED(ary) || OBJ_UNTRUSTED(sep)) untrust = TRUE;
 
diff --git a/test/ruby/test_array.rb b/test/ruby/test_array.rb
index 6724634691..e6a438cd73 100644
--- a/test/ruby/test_array.rb
+++ b/test/ruby/test_array.rb
@@ -890,6 +890,7 @@ class TestArray < Test::Unit::TestCase
     a = @cls[]
     assert_equal("", a.join)
     assert_equal("", a.join(','))
+    assert_equal(Encoding::US_ASCII, a.join.encoding)
 
     $, = ""
     a = @cls[1, 2]