Index: test/ruby/test_string.rb
===================================================================
--- test/ruby/test_string.rb	(revision 30814)
+++ test/ruby/test_string.rb	(working copy)
@@ -1308,6 +1308,18 @@ class TestString < Test::Unit::TestCase
     assert(a.untrusted?)
   end
 
+  def test_sub_bang_encoding
+    a = S("a")
+    a.force_encoding Encoding::BINARY
+
+    r = ' '
+    r.force_encoding Encoding::UTF_8
+
+    a.sub!(/a/) do r end
+
+    assert_equal Encoding::BINARY, a.encoding
+  end
+
   def test_succ
     assert_equal(S("abd"), S("abc").succ)
     assert_equal(S("z"),   S("y").succ)