Strange warning on overridden var args method

The following program produces warnings only on -Xlint:all.
However, the warning does not have a prefix to identify its
lint category.

import static java.lang.System.out;
class Base {
    void method1(Object[] args) {
        out.println("Base:method1");
        for (Object arg : args)
            out.println(arg);
    }

    void method2(Object... args) {
        out.println("Base:method2");
        for (Object arg : args)
            out.println(arg);
    }
}


class Derived extends Base {
    void method1(Object... args) {
        out.println("derived:method1");
        for (Object arg : args)
            out.println(arg);
    }

    void method2(Object[] args) {
        out.println("derived:method2");
        for (Object arg : args)
            out.println(arg);
    }
}
class Tester {
    public static void main(String[] args) {

        out.println("Working with Base");
        Base b = new Base();
        b.method1(new Object[] { "Hello", "World"});
        b.method2("Hello", "World");
        b.method2(new Object[] { "Hello", "World"});

        out.println("Working with Derivedd");
        Derived d = new Derived();
        d.method1("Hello", "World");
        d.method1(new Object[] { "Hello", "World"});
        d.method2("Hello", "World");
        d.method2(new Object[] { "Hello", "World"});

        out.println("Working with Derived as Base");
        b = d;
        b.method1(new Object[] { "Hello", "World"});
        b.method2("Hello", "World");
        b.method2(new Object[] { "Hello", "World"});


    }
}