LINQ weak relation between tables

I have two tables with a weak relation. I need get a text value from one table using a key from another. I am using the following C# LINQ code:

City = rea.tRealEstateContact.tPostnumre != null ? rea.tRealEstateContact.tPostnumre.Bynavn : string.Empty

But when the key cannot be found 开发者_运维技巧in the table 1(tPostnumre), an exception is thrown.

How should I do this?

Isn't there a tPostnumreID in the RealEstateContact table that's used to link to tPostnumre?

 City = rea.tRealEstateContact.tPostnumreID != null ?
          rea.tRealEstateContact.tPostnumre.Bynavn : string.Empty