add Index on a merge table

[DANIELE laurent]

I try to add one Index on a merge table
ALTER TABLE `TABLE_MERGE` ADD INDEX `Index_1`(`CODE`);
I try after create the Index on the merged table (with same name or différent name)
ALTER TABLE `TABLE1` ADD INDEX `Index_1`(`CODE`);
ALTER TABLE `TABLE2` ADD INDEX `Index_1`(`CODE`);
or and without this index

and I have always the error 'Mysql Error number 1168'.

could you help me...