验证在ASP.NET的失败一个或多个实体的实体框架

例外是当我试图转换后的图片保存为Base64格式：验证在ASP.NET的失败一个或多个实体的实体框架

public async Task<HttpResponseMessage> Upload() 
{ 
    if (!Request.Content.IsMimeMultipartContent()) 
    { 
     Request.CreateResponse(HttpStatusCode.UnsupportedMediaType); 
    } 

    var provider = GetMultipartProvider(); 

    var result = await Request.Content.ReadAsMultipartAsync(provider); 

    //Get Album name from Form 
    var titleOfAlbum = GetTitleOfAlbum(provider); 
    //get path to file 
    var pathToCoverDecoded = result.FileData.First().LocalFileName; 
    //ENCODE IMAGE TO BASE64 
    var bMap = new Bitmap(pathToCoverDecoded); 
    byte[] imageBtes; 
    string base64; 
    using (MemoryStream ms = new MemoryStream()) 
    { 
     bMap.Save(ms, bMap.RawFormat); 
     imageBtes = ms.ToArray(); 
     base64 = Convert.ToBase64String(imageBtes); 
    } 

    Album al = new Album(); 
    al.Title = titleOfAlbum; 
    al.PathToCover = base64; 
    db.Albums.Add(al); 
    db.SaveChanges(); 

    return new HttpResponseMessage(HttpStatusCode.OK); 
}

异常信息的截图