why Object reference not set to an instance of an object.?

why Object reference not set to an instance of an object.?

hi why this error is happened on **?
error : Object reference not set to an instance of an object.
public static long GetID(string str1, string str2)
{
using(MyContext db = new MyContext())
{
**long Idold = db.old.FirstOrDefault(p => p.oldfield ==
str1).id;**
return db.tbl_new.FirstOrDefault(p => p.id_old == Idold && p.newfield
== str2).id;
}
}
of course my tbl isn't empty .i set values to str1 and str2 that are exist
in my tbl.