What would prevent me from altering the structure of a readwrite cursor?

In two different locations in my application, I am creating a readwrite cursor and then dropping the memo fields because I later want to do a union and you can't union cursors with memo fields.

I know several work arounds, but I am puzzled since dropping a field works fine in one instance and is failing in another.

So here is some sample code:

* mhistory here is the memo field
select cfirst, clast, mhistory ;
 from customer ;
 where clast = "Brown" ;
 into cursor cur_selectedcustomers ;
 readwrite

alter table cur_selectedcustomers drop column mhistory

The error I am getting is: Invalid operation for the cursor (Error 1115)

Which from the VFP help is: An attempt was made to use the PACK command on a cursor.