What cause org.hibernate.PropertyAccessException: Exception occurred inside setter

What cause this exception, I can't manage to find out.

Request processing failed; nested exception is javax.persistence.PersistenceException: org.hibernate.PropertyAccessException: Exception occurred inside setter of my.Class

Root cause:

javax.persistence.PersistenceException: org.hibernate.PropertyAccessException: Exception occurred inside setter of my.Class

Code:

@ManyToMany(fetch = FetchType.EAGER)
public void setTags(Set<Tags> tags) {
    this.tags.clear();

    for (Tag tag : tags) {
        addTag(tag);
    }
}


public boolean addTag(final Tag tag) {
    if (tags.contains(tag)) {
        return false;
    }

    return tags.add(tag);
}

I initalize tags in the constructor:

tags = new HashSet<Tag>();

EDIT

Exception from logging in setter method:

ax.persistence.PersistenceException: org.hibernate.PropertyAccessException: Exception occurred inside setter of com.mycompany.domain.Book.Tags
    at org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1387) ~[hibernate-entitymanager-4.2.0.Final.jar:4.2.0.Final]
    at org.hibernate.ejb.AbstractEntityManagerImpl.find(AbstractEntityManagerImpl.java:838) ~[hibernate-entitymanager-4.2.0.Final.jar:4.2.0.Final]
    at org.hibernate.ejb.AbstractEntityManagerImpl.find(AbstractEntityManagerImpl.java:781) ~[hibernate-entitymanager-4.2.0.Final.jar:4.2.0.Final]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:201304051638]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) ~[na:201304051638]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:201304051638]
    at java.lang.reflect.Method.invoke(Method.java:601) ~[na:1.7.0]
    at org.springframework.orm.jpa.SharedEntityManagerCreator$SharedEntityManagerInvocationHandler.invoke(