diff --git a/ChangeLog b/ChangeLog
index b9f73f9232..7a2ed0738b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Aug  3 07:46:30 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* encoding.c (enc_find): mistakenly remained !.  [Bug #5150]
+
 Wed Aug  3 00:11:08 2011  Tanaka Akira  <akr@fsij.org>
 
 	* lib/prettyprint.rb: update document.  [ruby-core:36776]
diff --git a/encoding.c b/encoding.c
index 8e80e8ce66..e5bc75b4ef 100644
--- a/encoding.c
+++ b/encoding.c
@@ -1054,7 +1054,7 @@ static VALUE
 enc_find(VALUE klass, VALUE enc)
 {
     int idx;
-    if (!SPECIAL_CONST_P(enc) && BUILTIN_TYPE(enc) == T_DATA && is_data_encoding(enc))
+    if (RB_TYPE_P(enc, T_DATA) && is_data_encoding(enc))
 	return enc;
     idx = str_to_encindex(enc);
     if (idx == UNSPECIFIED_ENCODING) return Qnil;
diff --git a/test/ruby/test_encoding.rb b/test/ruby/test_encoding.rb
index 6ddac1d51a..c1c50a5dfa 100644
--- a/test/ruby/test_encoding.rb
+++ b/test/ruby/test_encoding.rb
@@ -50,6 +50,9 @@ class TestEncoding < Test::Unit::TestCase
         exit Encoding.find("filesystem") == Encoding::EUC_JP
       EOS
     end
+
+    bug5150 = '[ruby-dev:44327]'
+    assert_raise(TypeError, bug5150) {Encoding.find(1)}
   end
 
   def test_replicate