diff --git a/csharp/src/Google.Protobuf/Collections/MapField.cs b/csharp/src/Google.Protobuf/Collections/MapField.cs
index 924743e5af..4865e50aef 100644
--- a/csharp/src/Google.Protobuf/Collections/MapField.cs
+++ b/csharp/src/Google.Protobuf/Collections/MapField.cs
@@ -500,6 +500,7 @@ namespace Google.Protobuf.Collections
             WriteTo(ref ctx, codec, listToWrite);
         }
 
+        [SecuritySafeCritical]
         private void WriteTo(ref WriteContext ctx, Codec codec, IEnumerable<KeyValuePair<TKey, TValue>> listKvp)
         {
             foreach (var entry in listKvp)