Linq to SQL and UNION - "All queries combined using a UNION, INTERSECT or EXCEPT operator must have an equal number of expressions in their target lists."

I'm trying to link together quite a few different tables at the moment in Linq...

And I've hit an interesting SQL error....

"All queries combined using a UNION, INTERSECT or EXCEPT operator must have an equal number of expressions in their target lists."

Bascially what is happening is:

The two queries I'm trying to UNION are:

                var inwardActivityCommentFeedItems = from aComment in db.SlodgeComments_ActivityComments
                                                join comment in db.SlodgeComments_Comments on aComment.CommentId equals comment.CommentId
                                                join activity in db.SlodgeRoutes_Activities on aComment.ActivityId equals activity.ActivityId
                                                join activityUser in db.dnn_Users on activity.UserId equals activityUser.UserID
                                                join commentUser in db.dnn_Users on comment.CommenterUserId equals commentUser.UserID
                                                where activityUser.UserID == userId
                                                orderby comment.CommentDate descending
                                                select new FeedItem
                                                {
                                                    ActivityId = activity.ActivityId,
                                                    Date = comment.CommentDate,
                                                    CommentingUserId = commentUser.UserID,
                                                    CommentingUserName = commentUser.Username,
                                                    CommentText = comment.CommentText,
                                                    FeedItemType = FeedItemType.ActivityComment,
                                                    ActivityTitle = activity.Title,
                                                    TargetUserId = activityUser.UserID,
                                                    CommentId = comment.CommentId,
                                                    TargetUserName = activityUser.Username                                           
                                                };

                var activityFeedItems = from activity in db.SlodgeRoutes_Activities
                                      join user in db.dnn_Users on activity.UserId equals user.UserID
                                      where activity.UserId == userId
                                      orderby activity.StartTime descending
                                      select new FeedItem
                                      {
                                          ActivityId = activity.ActivityId,
                                          ActivityTitle = activity.Title,
                                          Date = activity.StartTime,
                                          CommentingUserId = -1,
                                          CommentingUserName = string.Empty,
                                          CommentText = string.Empty,
                                          CommentId = -1,
                                          FeedItemType = FeedItemType.Activity,
                                          TargetUserId = user.UserID,
                                          TargetUserName = user.Username
                                      };

So they both produce FeedItems...

But the SQL produced is:

{SELECT TOP (40) [t8].[value] AS [FeedItemType], [t8].[UserID] AS [CommentingUserId], [t8].[Username] AS [CommentingUserName], [t8].[UserID2] AS [TargetUserId], [t8].[Username2] AS [TargetUserName], [t8].[CommentText], [t8].[CommentDate] AS [Date], [t8].[ActivityId], [t8].[Title] AS [ActivityTitle], [t8].[CommentId]
FROM (
    SELECT [t7].[value], [t7].[UserID], [t7].[Username], [t7].[UserID2], [t7].[Username2], [t7].[CommentText], [t7].[CommentDate], [t7].[ActivityId], [t7].[Title], [t7].[CommentId]
    FROM (
        SELECT @p1 AS [value], [t4].[UserID], [t4].[Username], [t3].[UserID] AS [UserID2], [t3].[Username] AS [Username2], [t1].[CommentText], [t1].[CommentDate], [t2].[ActivityId], [t2].[Title], [t1].[CommentId]
        FROM [dbo].[SlodgeComments_ActivityComment] AS [t0]
        INNER JOIN [dbo].[SlodgeComments_Comment] AS [t1] ON [t0].[CommentId] = [t1].[CommentId]
        INNER JOIN [dbo].[SlodgeRoutes_Activities] AS [t2] ON [t0].[ActivityId] = [t2].[ActivityId]
        INNER JOIN [dbo].[dnn_Users] AS [t3] ON [t2].[UserId] = [t3].[UserID]
        INNER JOIN [dbo].[dnn_Users] AS [t4] ON [t1].[CommenterUserId] = [t4].[UserID]
        WHERE [t3].[UserID] = @p0
        UNION
        SELECT @p3 AS [value], @p4 AS [value2], @p5 AS [value3], [t6].[UserID], [t6].[Username], [t5].[StartTime], [t5].[ActivityId], [t5].[Title]
        FROM [dbo].[SlodgeRoutes_Activities] AS [t5]
        INNER JOIN [dbo].[dnn_Users] AS [t6] ON [t5].[UserId] = [t6].[UserID]
        WHERE [t5].[UserId] = @p2
        ) AS [t7]
    ) AS [t8]
}

If you look closely at the SQL you can see the two selection statements inside the union have been generated as:

        SELECT @p1 AS [value],
[t4].[UserID],
[t4].[Username],
[t3].[UserID] AS [UserID2],
[t3].[Username] AS [Username2],
[t1].[CommentText],
[t1].[CommentDate],
[t2].[ActivityId],
[t2].[Title],
[t1].[CommentId]

        SELECT @p3 AS [value],
@p4 AS [value2],
@p5 AS [value3],
[t6].[UserID],
[t6].[Username],
[t5].[StartTime],
[t5].[ActivityId],
[t5].[Title]

So the second statement does have less SELECT items returned than the first.

....


Eventually I worked this out as.... the problem is in bit of the the Linq which says:
                                          CommentingUserId = -1,
                                          CommentingUserName = string.Empty,
                                          CommentText = string.Empty,
                                          CommentId = -1,

The SQL generated by this somehow decides it can cut down on returning the -1's twice and the string.empty's twice...

So if I replace this with
                                          CommentingUserId = -1,
                                          CommentingUserName = "1",
                                          CommentText = "2",
                                          CommentId = -2,

Then I no longer see the error....

Of course I'm not saying this Linq is optimal or good practice - I'm just coding at the moment :)