Search text contains with QueryOver

I'm trying to do this :

var list = Session.QueryOver<Person>()
    .Where(x => x.LastName.Contains(searchText))
    .List<Person>();

but I get this error : Unrecognised method call: System.String:Boolean Contains(System.String)

Do you have an idea ?

Update :

public class Person
{
    public virtual string FirstName { get; set; }
    public virtual string LastName { get; set; }
}