I have a piece of code that throws exception and I dont quite understand why.

 public  async Task<List<CategoryVm>> Handle(GetCategoriesListQuery request, CancellationToken cancellationToken)
    {
        var categories = (await _categoryRepository.ListAllAsync()).OrderBy(x => x.Name);
      //  var list = categories.ToList();
        return _mapper.Map<List<CategoryVm>>(categories);
    }

I managed to get around problem as below. Would be be nice know why this works.

  public  async Task<List<CategoryVm>> Handle(GetCategoriesListQuery request, CancellationToken cancellationToken)
    {
        var categories = (await _categoryRepository.ListAllAsync()).OrderBy(x => x.Name);
        var list = categories.ToList();
        return _mapper.Map<List<CategoryVm>>(list);
    }

Category and CategoryVm

public class CategoryVm
{
    public Guid CategoryId { get; set; }
    public string Name { get; set; } 
}
public class Category: AuditableEntity
{
    public Guid CategoryId { get; set; }
    public string Name { get; set; }
    public ICollection<Event> Events { get; set; }
}