diff --git a/betterproto-extras/src/merging.rs b/betterproto-extras/src/merging.rs
index 7f4d86a..6f13084 100644
--- a/betterproto-extras/src/merging.rs
+++ b/betterproto-extras/src/merging.rs
@@ -14,6 +14,19 @@ pub fn merge_msg_into_pyobj(obj: &PyAny, mut msg: DynamicMessage) -> Result<()>
             map_field_value(field_name, field.1, proto_meta)?,
         )?;
     }
+
+    let mut buf = vec![];
+    for field in msg.unknown_fields() {
+        field.encode(&mut buf);
+    }
+    if !buf.is_empty() {
+        let mut unknown_fields = obj.getattr("_unknown_fields")?.extract::<Vec<u8>>()?;
+        unknown_fields.append(&mut buf);
+        obj.setattr("_unknown_fields", unknown_fields)?;
+    }
+
+    obj.setattr("_serialized_on_wire", true)?;
+
     Ok(())
 }