diff --git a/ChangeLog b/ChangeLog
index ac6a27621b..3f7ab2004f 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Thu Apr 24 23:17:25 2014  NAKAMURA Usaku  <usa@ruby-lang.org>
+
+	* lib/fileutils.rb (fu_get_uid, fu_get_gid): Etc.getpwnam/getgrnam may
+	  returns nil.
+
+	* lib/webrick/utils.rb (su): ditto.
+
 Thu Apr 24 22:55:22 2014  Tanaka Akira  <akr@fsij.org>
 
 	* bootstraptest/test_io.rb: Add etc.so to $" before require 'tmpdir'.
diff --git a/lib/fileutils.rb b/lib/fileutils.rb
index 1b7bb11b18..0dfbf41d6c 100644
--- a/lib/fileutils.rb
+++ b/lib/fileutils.rb
@@ -1106,7 +1106,7 @@ module FileUtils
       when /\A\d+\z/
         user.to_i
       else
-        Etc.getpwnam(user).uid
+        Etc.getpwnam(user) ? Etc.getpwnam(user).uid : nil
       end
     end
     private_module_function :fu_get_uid
@@ -1119,7 +1119,7 @@ module FileUtils
       when /\A\d+\z/
         group.to_i
       else
-        Etc.getgrnam(group).gid
+        Etc.getgrnam(group) ? Etc.getgrnam(group).gid : nil
       end
     end
     private_module_function :fu_get_gid
diff --git a/lib/webrick/utils.rb b/lib/webrick/utils.rb
index a6b5cc6a9c..185b1723f3 100644
--- a/lib/webrick/utils.rb
+++ b/lib/webrick/utils.rb
@@ -41,8 +41,7 @@ module WEBrick
     ##
     # Changes the process's uid and gid to the ones of +user+
     def su(user)
-      if defined?(Etc)
-        pw = Etc.getpwnam(user)
+      if defined?(Etc) && (pw = Etc.getpwnam(user))
         Process::initgroups(user, pw.gid)
         Process::Sys::setgid(pw.gid)
         Process::Sys::setuid(pw.uid)