Commons Lang、Validate.notEmpty()

import java.util.*;
import org.apache.commons.lang.Validate;

public class C2009051700 {
    public static void main(String[] args) {
        ArrayList a = new ArrayList();
        a.add("foo"); a.add("bar"); a.add("baz");

        Validate.notEmpty(a, "foo");

        a = new ArrayList();
        Validate.notEmpty(a, "foo");
    }
}

で、

Exception in thread "main" java.lang.IllegalArgumentException: foo
        at org.apache.commons.lang.Validate.notEmpty(Validate.java:281)
        at C2009051700.main(C2009051700.java:13)