Cast Exception When Try To Insert Entity In Entity Framework (using Code-f)

I get an cast exception when i am trying to insert an entity in Entity Framework (using code-first). From this code :

public virtual T Insert(T entity)
{
return Context.Set<T>().Add(entity);
}

The cast exception is like "impossible to cast ...Collection'1(Entity) to type (Entity)" I can't figure out why. I am pretty sure ive done everything right. Post entity

public class Post
{
public long PostId { get; private set; }
public DateTime date { get; set; }
[Required]
public string Subject { get; set; }
public User User { get; set; }
public Category Category { get; set; }
[Required]
public string Body { get; set; }
public virtual ICollection<Tag> Tags { get; private set; }
public Post()
{
Category = new Category();
if (Tags == null)
Tags = new Collection<Tag>();
}................................