Getting Null value when there is no null in Database table

So what i am doing is that i am taking the response from a api endpoint, saving it to my DB table and then displaying the table contents.

I have gone through possible reasons as to why one can get a null value, as explained here http://www.sql-server-helper.com/error-messages/msg-515.aspx, and none of those reasons apply in this case.

For this particular table, i am getting a exception error,

An error occurred while updating the entries. See the inner exception for details.

Here is the exception message i get:

An error has occurred. Cannot insert the value NULL into column 'FrameSizeID', table 'diafirearmserver.dbo.Reference_Frame_Sizes'; column does not allow nulls. INSERT fails. The statement has been terminated. System.Data.SqlClient.SqlException at System.Data.SqlClient.SqlCommand.<>c.b__167_0(Task1 result) at System.Threading.Tasks.ContinuationResultTaskFromResultTask2.InnerInvoke() at System.Threading.Tasks.Task.Execute() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Data.Entity.Core.Mapping.Update.Internal.DynamicUpdateCommand.d__0.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at System.Data.Entity.Core.Mapping.Update.Internal.UpdateTranslator.d__0.MoveNext()

My Questions is: Why am i getting a null value being passed, when the table (Screenshot below) has no null values in it? What am i overlooking?

Here is my data model:

namespace FirearmsAPI.Models
{
    using System;
    using System.Collections.Generic;

    public partial class Reference_Frame_Sizes
    {
        public int FrameSizeID { get; set; }
        public string FrameSize { get; set; }
    }
}

Here is my Controller:

public class Reference_Frame_SizesController : ApiController
{
    private DataEntities db = new DataEntities();
    static string _address = "http://localhost:57454/api/Reference_Frame_Sizes?format=json";
    private List<Reference_Frame_Sizes> result;

    // GET: api/Reference_Frame_Sizes
    /// <summary>
    /// Retrieves Firearm Frame size data from external api endpoint, saves data to DB and returns Data
    /// </summary>
    public async Task<IEnumerable<Reference_Frame_Sizes>> GetReference_Frame_Sizes()
    {
        db.Database.ExecuteSqlCommand("TRUNCATE TABLE Reference_Frame_Sizes"); //Truncates internal table

        List<Reference_Frame_Sizes> resultset = await GetResponse();
        foreach (Reference_Frame_Sizes manu in resultset)
        {
            db.Reference_Frame_Sizes.Add(manu);
        }
        await db.SaveChangesAsync();
        return db.Reference_Frame_Sizes;
    }

    private async Task<List<Reference_Frame_Sizes>> GetResponse()
    {
        var client = new HttpClient();
        HttpResponseMessage response = await client.GetAsync(_address);
        response.EnsureSuccessStatusCode();
        result = await response.Content.ReadAsAsync<List<Reference_Frame_Sizes>>();
        return result;
    }

protected override void Dispose(bool disposing)
    {
        if (disposing)
        {
            db.Dispose();
        }
        base.Dispose(disposing);
    }

    private bool Reference_Frame_SizesExists(int id)
    {
        return db.Reference_Frame_Sizes.Count(e => e.FrameSizeID == id) > 0;
    }
}