The "X" property on "Y" could not be set to a 'null' value. You must set this property to a non-null value of type 'Int32'

 "The "X" property on "Y" could not be set to a 'null' value. You must set this property to a non-null value of type 'Int32'."

In your EDMX, if you go under your Y table and click on X column, right-click, click on Properties, scroll down to Nullable and change from False to True.

If you get a "mapping fragment" error, you'll have to delete the table from the EDMX and re-add it, because in the Model Browser it stores the table properties and the only way to refresh that (that I know of) is to delete the table from the Model Browser under <database>.Store then retrieving it using Update Model from Database.. command.

I just replace data type int to int32?

public Int32 Field{ get; set; }

to

public Int32? Field{ get; set; }

and the problem is solved

My problem was that my Model database was out of sync with the actual (dev) database. So the EDMX thought it was smallint but the actual column was int. I updated the model database to int and the EDMX to Int32 and now it works.