"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.