diff --git a/ChangeLog b/ChangeLog
index cda5e6a3c3..ee77f9439a 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+Sat Jan  5 13:38:07 2013  Masaki Matsushita  <glass.saga@gmail.com>
+
+	* string.c (rb_str_enumerate_lines): fix invalid byte sequence error
+	  when a separator is passed. The patch is from yoshidam (Yoshida
+	  Masato).
+	  [Bug #7646] [ruby-dev:46827]
+
+	* test/ruby/test_string.rb: a test for above.
+
 Sat Jan  5 12:25:42 2013  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* test/ruby/envutil.rb (Test::Unit::Assertions#assert_in_out_err):
diff --git a/string.c b/string.c
index a3f937d19e..d0c30c6c7b 100644
--- a/string.c
+++ b/string.c
@@ -6199,14 +6199,14 @@ rb_str_enumerate_lines(int argc, VALUE *argv, VALUE str, int wantarray)
 	if (c == newline &&
 	    (rslen <= 1 ||
 	     (pend - p >= rslen && memcmp(RSTRING_PTR(rs), p, rslen) == 0))) {
-	    p += (rslen ? rslen : n);
-	    line = rb_str_subseq(str, s - ptr, p - s);
+	    const char *pp = p + (rslen ? rslen : n);
+	    line = rb_str_subseq(str, s - ptr, pp - s);
 	    if (wantarray)
 		rb_ary_push(ary, line);
 	    else
 		rb_yield(line);
 	    str_mod_check(str, ptr, len);
-	    s = p;
+	    s = pp;
 	}
 	p += n;
     }
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index e988f99629..7b4dc2951d 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -760,6 +760,11 @@ class TestString < Test::Unit::TestCase
 
     assert_equal "hello\n", S("hello\nworld").each_line.next
     assert_equal "hello\nworld", S("hello\nworld").each_line(nil).next
+
+    bug7646 = "[ruby-dev:46827]"
+    assert_nothing_raised(bug7646) do
+      "\n\u0100".each_line("\n") {}
+    end
   end
 
   def test_lines