InvalidOperationException: The entity type 'Array' requires a primary key to be defined

I am making a Web API with .Net and it is receiving a custom JSON object from a web form to serialize to the model. I am currently having an issue displaying the API response. I am storing the data in an in memory database at first just to get it working.

I am not sure why it is asking me for a primary key despite there being a key defined on my model.

My next step is to add multipart-form data but that is its own can of worms.
The error happens on the _context.ProjectItems.ToListAsync(); line.
I have already added an ID to the model.  


    [Required]
    [Key]
    public new long Id { get; set; }

    [HttpGet("{id}")]
    public async Task<ActionResult<ProjectItem>> GetProjectItem(long id)
    {
        var projectItem = await _context.ProjectItems.FindAsync(id);

    if (projectItem == null)
    {
        return NotFound();
    }

return projectItem;

   }
  [HttpGet]
    public async Task<ActionResult<IEnumerable<ProjectItem>>> 
GetProjectItems()
    {
        return await _context.ProjectItems.ToListAsync();
    }

My model: ProjectItem.cs using System.ComponentModel.DataAnnotations;

    namespace RupAPI.Controllers
    {
        public class ProjectItem : Project
        {
            [Required]
            [Key]
            public new long Id { get; set; }

            public string ProjName { get; set; }
            public DateTime DateSub { get; set; }
            public DateTime DueDate { get; set; }
            public DateTime ArrivalDate { get; set; }
            public string Desc { get; set; }
            public Array Sku { get; set; }
            public Array SkuDesc { get; set; }
            public Array Item { get; set; }
            public Array ItemDesc { get; set; }
            public Array FileName { get; set; }
            public new byte[] File { get; set; }
        }
    }

An unhandled exception occurred while processing the request. InvalidOperationException: The entity type 'Array' requires a primary key to be defined.
    Microsoft.EntityFrameworkCore.Infrastructure.ModelValidator.ValidateNonNullPrimaryKeys(IModel model)
    Stack Query Cookies Headers 
    InvalidOperationException: The entity type 'Array' requires a primary key to be defined.
    +
                return await _context.ProjectItems.ToListAsync();
    lambda_method(Closure , object )