diff --git a/ChangeLog b/ChangeLog
index 3a47b885b5..c7d6b6754b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Mon Dec 15 16:19:23 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* string.c (rb_enc_str_coderange): dummy wchar, non-endianness
+	  encoding string cannot be ascii only.
+	  [ruby-core:66835] [Bug #10598]
+
 Sun Dec 14 20:11:42 2014  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* parse.y (primary): restore current_arg so that circular
diff --git a/string.c b/string.c
index 9006abdfac..9053835ee2 100644
--- a/string.c
+++ b/string.c
@@ -487,8 +487,15 @@ rb_enc_str_coderange(VALUE str)
     int cr = ENC_CODERANGE(str);
 
     if (cr == ENC_CODERANGE_UNKNOWN) {
-	rb_encoding *enc = STR_ENC_GET(str);
-        cr = coderange_scan(RSTRING_PTR(str), RSTRING_LEN(str), enc);
+	int encidx = ENCODING_GET(str);
+	rb_encoding *enc = rb_enc_from_index(encidx);
+	if (rb_enc_mbminlen(enc) > 1 && rb_enc_dummy_p(enc)) {
+	    cr = ENC_CODERANGE_BROKEN;
+	}
+	else {
+	    cr = coderange_scan(RSTRING_PTR(str), RSTRING_LEN(str),
+				get_actual_encoding(encidx, str));
+	}
         ENC_CODERANGE_SET(str, cr);
     }
     return cr;
diff --git a/test/ruby/test_m17n.rb b/test/ruby/test_m17n.rb
index bbaf31f52a..99450723cf 100644
--- a/test/ruby/test_m17n.rb
+++ b/test/ruby/test_m17n.rb
@@ -260,6 +260,14 @@ class TestM17N < Test::Unit::TestCase
     end
   end
 
+  def test_utf_without_bom_asciionly
+    bug10598 = '[ruby-core:66835] [Bug #10598]'
+    encs = [Encoding::UTF_16, Encoding::UTF_32].find_all {|enc|
+      "abcd".force_encoding(enc).ascii_only?
+    }
+    assert_empty(encs, bug10598)
+  end
+
   def test_object_utf16_32_inspect
     EnvUtil.suppress_warning do
       begin