Why do I receive NullReferenceException if I use a Nullable Entity Framework lazy-loading referenced field?

Today I come across a very strange error. I have the following method:

    public IQueryable<AspNetUserDetails> GetDetails()
    {
        return this.Get()
            .Include(q => q.AspNetRoles)
            .OrderBy(q => q.UserName)
            .Select(q => new AspNetUserDetails()
            {
                AspNetUser = q,
                Roles = q.AspNetRoles,
                StoreAdmin = q.Store,
            });
    }

(in the database, AspNetUser has AdminStoreId column that allows null, and is a foreign key to primary key of table Store)

In my caller, I have this code to convert to ViewModel (it has the same field names as the Entity):

    public async Task<IEnumerable<AspNetUserDetailsViewModel>> GetDetails()
    {
        return await this.BaseService.GetDetails()
            .ProjectTo<AspNetUserDetailsViewModel>(this.AutoMapperConfig)
            .ToListAsync();
    }

I recieved NullReferenceException at the ProjectTo method. The ProjectTo method is of AutoMapper, and the problem also happens if I use normal Select statement, so it has nothing to do with AutoMapper. I have already solved the problem by changing the Entity Select statement to this:

            .Select(q => new AspNetUserDetails()
            {
                AspNetUser = q,
                Roles = q.AspNetRoles,
                StoreAdmin = q.AdminStoreId == null ? null : q.Store,
            });

My question is: why? Shouldn't q.Store is already null if it has no reference?