diff --git a/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
index f9b90619adb54..51d87353135dc 100644
--- a/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs
@@ -239,7 +239,8 @@ public bool IsPacked
                 }
                 else
                 {
-                    return !Proto.Options.HasPacked || Proto.Options.Packed;
+                    // Packed by default with proto3
+                    return Proto.Options == null || !Proto.Options.HasPacked || Proto.Options.Packed;
                 }
             }
         }