Deleting records in LINQ with foreign keys - invalid cast exception

I have a simple database:

ARTICLE
----------
ArticleId (PK),
ArticleTitle
..other stuff...

USER-ARTICLE
------------
ArchiveId (PK),
UserId,
ArticleId
..other stuff...

The articleId's are foreign keys.

I want to be able to delete a user article row by UserArticleId using the following code

  UserArticle myobjtodelete = PersonalArchiveDb.UserArticles.Single(ua => ua.ArchiveId == 3);

        PersonalArchiveDb.UserArticles.DeleteOnSubmit(myobjtodelete);
        PersonalArchiveDb.SubmitChanges();

(yes i am aware i can do the statement inside the delete rather than retrieving the object, it was for debugging to make sure the object definitly exists - it does)

When the debugger hits the SubmitChanges() line, I get a runtime error:

Specified cast is not valid.

Here is the stack trace

at System.Data.Linq.IdentityManager .StandardIdentityManager .SingleKeyManager\2.TryCreateKeyFromValues(Object[] values, V& v) at System.Data.Linq.IdentityManager.StandardIdentityManager.IdentityCache2.Find(Object[] keyValues) at System.Data.Linq.IdentityManager.StandardIdentityManager.Find(MetaType type, Object[] keyValues) at System.Data.Linq.CommonDataServices.GetCachedObject(MetaType type, Object[] keyValues) at System.Data.Linq.ChangeProcessor.GetOtherItem(MetaAssociation assoc, Object instance) at System.Data.Linq.ChangeProcessor.BuildEdgeMaps() at System.Data.Linq.ChangeProcessor.SubmitChanges(ConflictMode failureMode) at System.Data.Linq.DataContext.SubmitChanges(ConflictMode failureMode) at System.Data.Linq.DataContext.SubmitChanges() at Driver_SOC_ASO.Controls.PersonalArchive.ArchiveListing.grdArchive_RowDeleting(Object sender, GridViewDeleteEventArgs e) in C:\work\Driver.Net\Driver-SOC-ASO\Driver-SOC-ASO\Controls\PersonalArchive\ArchiveListing.ascx.cs:line 78 at System.Web.UI.WebControls.GridView.OnRowDeleting(GridViewDeleteEventArgs e) at System.Web.UI.WebControls.GridView.HandleDelete(GridViewRow row, Int32 rowIndex) at System.Web.UI.WebControls.GridView.HandleEvent(EventArgs e, Boolean causesValidation, String validationGroup) at System.Web.UI.WebControls.GridView.RaisePostBackEvent(String eventArgument) at System.Web.UI.WebControls.GridView.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

I am at a loss, any ideas?

From stackoverflow

• Your user-article table may be updated and you forgot to update the dbml?

  qui : No, that's not the case.

• try setting ON CASCADE DELETE for ArticleId

• This may be an example of this bug, which Microsoft says is fixed in .NET 4.0.

• Is ArticleId an integer, or compatible with it?

• In your dbml, check to see if there is incorrect an association b/w Article and UserArticle.