Why @OneToMany does not work with inheritance in Hibernate

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public class Problem {
    @ManyToOne
    private Person person;
}

@Entity
@DiscriminatorValue("UP")
public class UglyProblem extends Problem {}

@Entity
public class Person {
    @OneToMany(mappedBy="person")
    private List< UglyProblem > problems;
}

_____________
          |__PROBLEMS__|          |__PEOPLE__|
          |id <PK>     |          |          |
          |person <FK> | -------->|          |
          |problemType |          |_________ |
          --------------

public interface Problem {
    public Person getPerson();
}

public interface UglyProblem extends Problem {
}

@MappedSuperclass
public abstract class AbstractProblemImpl implements Problem {
    @ManyToOne
    private Person person;

    public Person getPerson() {
        return person;
    }
}

@Entity
public class ProblemImpl extends AbstractProblemImpl implements Problem {
}

@Entity
public class UglyProblemImpl extends AbstractProblemImpl implements UglyProblem {
}

public class A {
    private int foo;
}

@Entity
public class B extens A {
}

@Entity
public class Problem {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;

private String name;

public Long getId() {
	return id;
}

public void setId(Long id) {
	this.id = id;
}

public String getName() {
	return name;
}

public void setName(String name) {
	this.name = name;
}
}

@Entity
public class UglyProblem extends Problem {

private int levelOfUgliness;

public int getLevelOfUgliness() {
	return levelOfUgliness;
}

public void setLevelOfUgliness(int levelOfUgliness) {
	this.levelOfUgliness = levelOfUgliness;
}
}

CREATE TABLE "problem" (
    "DTYPE" varchar(31) NOT NULL,
    "id" bigint(20) NOT NULL auto_increment,
    "name" varchar(255) default NULL,
    "levelOfUgliness" int(11) default NULL,
    PRIMARY KEY  ("id")
) AUTO_INCREMENT=2;

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@ForceDiscriminator
public class Problem {

}

@Entity
@DiscriminatorValue("UP")
public class UglyProblem extends Problem {
    @ManyToOne
    private Person person;
}

@Entity
public class Person {
    @OneToMany(mappedBy="person")
    private List< UglyProblem > problems;
}