diff --git a/objectivec/GPBUnknownFieldSet.h b/objectivec/GPBUnknownFieldSet.h
index 5901f18..8837f4c 100644
--- a/objectivec/GPBUnknownFieldSet.h
+++ b/objectivec/GPBUnknownFieldSet.h
@@ -39,7 +39,8 @@
  * of a message that are unknown end up in an instance of this set. This only
  * applies for files declared with the "proto2" syntax.
  **/
-@interface GPBUnknownFieldSet : NSObject <NSCopying>
+__attribute__((objc_subclassing_restricted))
+@interface GPBUnknownFieldSet : NSObject<NSCopying>
 
 /**
  * Tests to see if the given field number has a value.