null from Server

From my server i stired the server values to datatable for temporary operation.If incase my column has null in my server then its populating an error .

String contactNo = (from DataRow dr1 in employeedetail.Rows where (string)dr1["EmployeeID"] == userid select (string)dr1["ContactNo"]).FirstOrDefault();

if the column value is null then the error is arising.I dont want to populate error.If value is not null then it will append to my string or else the value will be null.please help me to solve this.