diff --git a/common/values/custom_map_value.h b/common/values/custom_map_value.h
index c96a565bd..9e840e07f 100644
--- a/common/values/custom_map_value.h
+++ b/common/values/custom_map_value.h
@@ -263,7 +263,7 @@ class CustomMapValueInterface {
 
   struct Content {
     const CustomMapValueInterface* absl_nonnull interface;
-    google::protobuf::Arena* absl_nonnull arena;
+    google::protobuf::Arena* absl_nullable arena;
   };
 };