diff --git a/ChangeLog b/ChangeLog
index f7aed29a14..f6812d0ada 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Wed Mar 24 10:18:12 2010  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* file.c (file_expand_path): set length of string before calling
+	  rb_enc_check because rb_enc_check scans its content.
+	  This prevents warnings by valgrind.
+
 Tue Mar 23 23:58:51 2010  James Edward Gray II  <jeg2@ruby-lang.org>
 
 	* lib/csv.rb: Incorporating the fixes from the recent
diff --git a/file.c b/file.c
index 7d6d25e21a..5a0b16eabe 100644
--- a/file.c
+++ b/file.c
@@ -2849,11 +2849,14 @@ file_expand_path(VALUE fname, VALUE dname, int abs_mode, VALUE result)
 	    p = chompdirsep(skiproot(buf));
     }
     else {
+	size_t len;
 	b = s;
 	do s++; while (isdirsep(*s));
-	p = buf + (s - b);
+	len = s - b;
+	p = buf + len;
 	BUFCHECK(bdiff >= buflen);
-	memset(buf, '/', p - buf);
+	memset(buf, '/', len);
+	rb_str_set_len(result, len);
 	rb_enc_associate(result, rb_enc_check(result, fname));
     }
     if (p > buf && p[-1] == '/')