"Cell is Read Only" with inner join in "Select Edit Top 200 Rows" option

When I right-click on Table1 and click on "Select Edit Top 200 Rows" option, I can change values, add/delete records, etc. As soon as I add an INNER JOIN in the sql editor to Table2, the cells become read-only and I get message "Cell is Read Only". The only difference between both queries is that one has the inner join and the other hasn't. Both queries display the same exact columns (all the columns from Table1).

Thanks.

VM