Cannot implicitly convert type 'System.Guid?' to 'DataContracts.Market'

[Range]

I am getting the following errorCannot implicitly convert type 'System.Guid?' to 'DataContracts.Market' \[code\] private CellSite MapEntityToCellSitePOCO(t_CellSite _cellsite) { CellSite cellsite= new CellSite(); cellsite.SiteId = _cellsite.SiteID; cellsite.Market.MarketID = _cellsite.MarketId; cellsite.Region.RegionId = _cellsite.RegionId; return cellsite; }\[/code\]the following is my datacontracts file\[code\] public class CellSite{ public Guid CellSiteID { get; set; } public string SiteId { get; set; } public Region Region { get; set; } public Market Market { get; set; } public Guid? ConstructionManager { get; set;} }\[/code\]This is market.cs\[code\] public class Market{ public Guid? MarketID { get; set; } public string OperatorMarketName { get; set; } public string MarketName { get; set; } public decimal AllOtherAmount { get; set; } public decimal RawLandAmount { get; set; } public decimal RenewalFee { get; set; } public bool IsActive { get; set; }}\[/code\]there is a column MarketId in cellsite table which i want to bring on.how can i do that? I am new to entity frame work.thanks in advance