diff --git a/ChangeLog b/ChangeLog
index 572046af74..81e0f5b5cc 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Wed Feb  3 20:08:05 2010  Tanaka Akira  <akr@fsij.org>
+
+	* file.c (realpath_rec): rb_path_last_separator may return NULL.
+
 Wed Feb  3 13:15:24 2010  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* thread_pthread.c: DragonFlyBSD is also the same as FreeBSD
diff --git a/file.c b/file.c
index ed8d27af17..e89a3cd864 100644
--- a/file.c
+++ b/file.c
@@ -3097,7 +3097,8 @@ realpath_rec(long *prefixlenp, VALUE *resolvedp, char *unresolved, VALUE loopche
         else if (testnamelen == 2 && testname[0] == '.' && testname[1] == '.') {
             if (*prefixlenp < RSTRING_LEN(*resolvedp)) {
                 char *resolved_names = RSTRING_PTR(*resolvedp) + *prefixlenp;
-                long len = rb_path_last_separator(resolved_names) - resolved_names;
+                char *lastsep = rb_path_last_separator(resolved_names);
+                long len = lastsep ? lastsep - resolved_names : 0;
                 rb_str_set_len(*resolvedp, *prefixlenp + len);
             }
         }