ddlcache values coming as null in Coupon service

I am using Amasty Multiple coupon extensions in our website. Its working fine. I would like to use this extension as REST API web service instead of use in the theme.

So we tried to use this extension as REST API web service and tried to use that. In that while fetching the data we are getting the below value a empty "[_ddlCache:protected] => Array
(
) "

Please suggest , why the _ddlcache array value coming as empty. Actually "sales_flat_order_grid " value has to come in the _ddlcache array.