Commons Lang、Validate.notEmpty()

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

public class C2009051900 {
    public static void main(String[] args) {
        Map m = new HashMap();
        m.put("foo", 1);

        Validate.notEmpty(m, "foo");

        m = new HashMap();
        Validate.notEmpty(m, "foo");
    }
}

で、

Exception in thread "main" java.lang.IllegalArgumentException: foo
        at org.apache.commons.lang.Validate.notEmpty(Validate.java:321)
        at C2009051900.main(C2009051900.java:13)