diff --git a/ChangeLog b/ChangeLog
index 3087a4cd95..8ce4a133dc 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Tue Dec 22 15:21:11 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (str_compat_and_valid): as scrub does nothing for dummy
+	  encoding string now, incompatible encoding is not a matter.
+
 Tue Dec 22 14:31:28 2015  Toru Iwase  <tietew@tietew.net>
 
 	* ext/cgi/escape/escape.c (optimized_escape_html): CGI.escapeHTML
diff --git a/string.c b/string.c
index 4ab89a8e2f..4f9fe51f9f 100644
--- a/string.c
+++ b/string.c
@@ -8639,18 +8639,11 @@ str_compat_and_valid(VALUE str, rb_encoding *enc)
     if (cr == ENC_CODERANGE_BROKEN) {
 	rb_raise(rb_eArgError, "replacement must be valid byte sequence '%+"PRIsVALUE"'", str);
     }
-    else if (cr == ENC_CODERANGE_7BIT) {
+    else {
 	rb_encoding *e = STR_ENC_GET(str);
-	if (!rb_enc_asciicompat(enc)) {
+	if (cr == ENC_CODERANGE_7BIT ? rb_enc_mbminlen(enc) != 1 : enc != e) {
 	    rb_raise(rb_eEncCompatError, "incompatible character encodings: %s and %s",
-		    rb_enc_name(enc), rb_enc_name(e));
-	}
-    }
-    else { /* ENC_CODERANGE_VALID */
-	rb_encoding *e = STR_ENC_GET(str);
-	if (enc != e) {
-	    rb_raise(rb_eEncCompatError, "incompatible character encodings: %s and %s",
-		    rb_enc_name(enc), rb_enc_name(e));
+		     rb_enc_name(enc), rb_enc_name(e));
 	}
     }
     return str;
diff --git a/test/ruby/test_m17n.rb b/test/ruby/test_m17n.rb
index 9f96652083..3a45f1f028 100644
--- a/test/ruby/test_m17n.rb
+++ b/test/ruby/test_m17n.rb
@@ -1642,6 +1642,12 @@ class TestM17N < Test::Unit::TestCase
                  scrub)
   end
 
+  def test_scrub_dummy_encoding
+    s = "\u{3042}".encode("iso-2022-jp")
+    assert_equal(s, s.scrub)
+    assert_equal(s, s.force_encoding("iso-2022-jp").scrub("?"))
+  end
+
   def test_scrub_bang
     str = "\u3042\u3044"
     assert_same(str, str.scrub!)