add method on a list of object doesn't add anythnig in this list

Hello,

I've a service task with an operation. This operation has the following script :

import com.company.model.MyObject
import java.util.logging.Logger
import java.util.ArrayList

Logger logger = Logger.getLogger("org.bonitasoft.groovy.script.myOperation");
logger.info("lines: " + myObject.getLines().toString());

myObject.lines.add(newLine);
logger.info("lines : "+myObject.getLines().toString());

return myObject;

The log before the .add displays an empty list that is normal but the log after the .add still displays an empty list.
I don't understand why the .add method doesn't add the new line in the list.

myObject is a pool variable initialize in a previous task and I haven't any errors in the logs

Somebody have an idea ?

Thanks by advance