Note that when the list of available Working tables is being assembled, WRDB does not actually examine the structure of each table to make sure that it is actually a valid WRDB data table (that would take too long). Instead, all tables in the database which have names different than the standard WRDB Support tables, and different from the current and all previously referenced Master tables, will be included in the list.
If you create a non-data table in the database using another tool (e.g., Microsoft Access), that table will appear in the working table list. When you try to open that table, the structure will be examined and you will receive an error message if it is not recognized as being valid.