diff --git a/ChangeLog b/ChangeLog
index 02f1cfa1a8..7c10c31d33 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Thu Jan  5 12:15:55 2012  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* file.c (rb_file_join): separator is appended by array length - 1
+	  times. patched by Benoit Daloze [ruby-core:41901] [Bug #5841]
+
 Thu Jan  5 11:47:54 2012  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* lib/uri/common.rb (URI::Parser#initialize_regexp):
diff --git a/file.c b/file.c
index fb62c51d7a..15d594a059 100644
--- a/file.c
+++ b/file.c
@@ -3825,7 +3825,7 @@ rb_file_join(VALUE ary, VALUE sep)
     }
     if (!NIL_P(sep)) {
 	StringValue(sep);
-	len += RSTRING_LEN(sep) * RARRAY_LEN(ary) - 1;
+	len += RSTRING_LEN(sep) * (RARRAY_LEN(ary) - 1);
     }
     result = rb_str_buf_new(len);
     OBJ_INFECT(result, ary);