When we try using the Kentico Upgrade tool to upgrade our sites to V7, we are receiving this error:

-- Update table [COM_Order]
UPDATE [COM_Order] SET
[OrderCreatedByUserID] = NULL
WHERE [OrderCreatedByUserID] = 0;

ALTER TABLE [COM_Order]
ADD
CONSTRAINT [FK_COM_Order_OrderCreatedByUserID_CMS_User]
FOREIGN KEY ([OrderCreatedByUserID]) REFERENCES [CMS_User] ([UserID]);

Failed to run SQLscript: The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK_COM_Order_OrderCreatedByUserID_CMS_User". The conflict occurred in database "BostwickCMS_V7", table "dbo.CMS_User", column 'UserID'.
Running SQL script (upgrade_6_0.sql)

Any help would be much appreciated.

Thanks,
Jon Wilson