Can't set the vaule of an attribute on a superclass

public class Test {
public String x = "hello";

public static void main(String[] args) {
final Test t1 = new Test();
final Test t2 = new Test() {}; // subclass of Test

InvokerHelper.getMetaClass(t1).setAttribute(t1, "x", "a");
InvokerHelper.getMetaClass(t2).setAttribute(t2, "x", "a");
}
}

produces:
Exception in thread "main" groovy.lang.MissingFieldException: No such field: x for class: Test$1
at groovy.lang.MetaClass.setAttribute(MetaClass.java:1138)
at Test.main(Test.java:35)