table invalid index

Hello everyone.

I have a webdynpro program which call a pdf form.

sometimes,some of our users are getting a dump "TABLE_INVALID_INDEX".

The error is on row which try to delete itab on specific index . (delete itab index lv_idx).

When the same pdf is being called from a regular sap program (not webdynpro), it runs OK.

Any ideas what can be the problem?

thanks