diff --git a/ChangeLog b/ChangeLog
index 965906200a..a55f39d53c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Sun May  6 14:50:03 2012  Tanaka Akira  <akr@fsij.org>
+
+	* lib/securerandom.rb: show actual read length in an error message.
+
 Sat May  5 06:43:10 2012  Ryan Davis  <ryand-ruby@zenspider.com>
 
 	* lib/minitest/*: Imported minitest 2.12.1 (r7323)
diff --git a/lib/securerandom.rb b/lib/securerandom.rb
index eede2fef04..a12e92fc7a 100644
--- a/lib/securerandom.rb
+++ b/lib/securerandom.rb
@@ -80,7 +80,7 @@ module SecureRandom
           @has_urandom = true
           ret = f.read(n)
           if ret.length != n
-            raise NotImplementedError, "Unexpected partial read from random device"
+            raise NotImplementedError, "Unexpected partial read from random device: only #{ret.length} for #{n}"
           end
           return ret
         }