Query Nullable Column Linq to Sql

I have a Nullable BIGINT field in the database table.

I am using linq to sql to retrieve data from that table in c#.

c.ExtensionID == (ExtensionId == 0 ? (long?)null : ExtensionId)

The above piece of code is what i am using currently, but it does not return me the rows with null in ExtensionID Column.

but if i use it like c.ExtensionID == null then it will return records.

Below is the full linq to sql code.

var q = (from c in dbContext.Investments
         join cd in dbContext.ContractContractorDetails
         on c.ContractContractorId equals cd.ID
         join cc in dbContext.ContractorCategories
         on c.ContractorCategory equals cc.ID
         join ic in dbContext.InvestmentCategories
         on c.InvestmentCategory equals ic.ID
         where c.InvestmentClassificationType == type && cd.ContractId.Equals(ContractID) && c.Phase == phase && c.ExtensionID == (ExtensionId == 0 ? (long?)null : ExtensionId)
         select new
         {
             ID = c.ID,
             Year = c.Year,
             ContractorCategory = c.ContractorCategory,
             ContractorCategoryName = cc.Name,
             CompanyName = c.CompanyName,
             InvestmentCategory = c.InvestmentCategory,
             InvestmentCategoryName = ic.Name,
             Summary = c.Summary,
             IsContractRelated = c.IsContractRelated,
             InvestmentAmount = c.InvestmentAmount,
             Phase = c.Phase,
             InvestmentClassificationType = c.InvestmentClassificationType,
             ContractContractorId = c.ContractContractorId,
             CreatedBy = c.CreatedBy,
             LastUpdatedOn = c.LastUpdatedOn,
             LastUpdatedBy = c.LastUpdatedBy,
             Period=c.Period,
             ExtensionId = c.ExtensionID
         }).ToList();