diff --git a/ChangeLog b/ChangeLog
index 6fa373a7a8..987fda39f2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Apr 20 14:41:28 2011  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* string.c (rb_str_each_line): check string's length when compare
+	  separator and string. [ruby-core:35815] fixes #4586
+
 Wed Apr 20 00:02:13 2011  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* misc/ruby-mode.el (ruby-parse-partial): use position of open paren.
diff --git a/string.c b/string.c
index 798e108d42..1f02ebff49 100644
--- a/string.c
+++ b/string.c
@@ -6051,7 +6051,8 @@ rb_str_each_line(int argc, VALUE *argv, VALUE str)
 	    p -= n;
 	}
 	if (c == newline &&
-	    (rslen <= 1 || memcmp(RSTRING_PTR(rs), p, rslen) == 0)) {
+	    (rslen <= 1 ||
+	     (pend - p >= rslen && memcmp(RSTRING_PTR(rs), p, rslen) == 0))) {
 	    line = rb_str_new5(str, s, p - s + (rslen ? rslen : n));
 	    OBJ_INFECT(line, str);
 	    rb_enc_cr_str_copy_for_substr(line, str);