Entity Framework: Issue with adding new entity to the contex after deleting

In my context I am deleting one of the existing entities. And in the same context I am trying to add a new entity with using code below:

        if (entity != null && EFContext.Entry(entity).State == EntityState.Detached)
        {
            try
            {
                var entityName = entity.GetType().Name;             
                GetObjectContext.AddObject(entityName + "s", entity);
            }
            catch (Exception ex)
            {
                log(ex);
                throw;
            }
        }

The problem I have is when I pass the new created entity to the method above, when it is checking the entity state, it throws exception: *

Adding a relationship with an entity which is in the Deleted state is not allowed

I haven't made any relationship with the deleted entity, why would it throw this exception? I am trying to reproduce the issue on another simple project, but can't get to reproduce it there. Does anyone know why would EF would throw that exception?

UPDATE: I have Student and a Backpack entity. Backpack references the Student(one to many). In my case I am deleting one of the backpacks and trying to create a new student entity. And when I am checking the object state I am getting the exception above.

Here is the call stack: