diff --git a/src/ProtocolBuffers/CustomSerialization.cs b/src/ProtocolBuffers/CustomSerialization.cs
index a658e95560..ae9fca2299 100644
--- a/src/ProtocolBuffers/CustomSerialization.cs
+++ b/src/ProtocolBuffers/CustomSerialization.cs
@@ -36,9 +36,9 @@
 /*
  * This entire source file is not supported on some platform
  */
-using System.Security.Permissions;
 #if !NOSERIALIZABLE
 using System;
+using System.Security.Permissions;
 using System.Runtime.Serialization;
 using System.Security;