HikaShop Authorize.net Error When Processing Order

-- HikaShop version -- : 2.3.5
-- Joomla version -- : 3.2.3
-- Browser(s) name and version -- : Firefox
-- Error-message(debug-mod must be tuned on) -- : When submitting order via Authorize.net HikaShop Plugin, we get the following error:

There was an error while processing your transaction: 33 (TESTMODE) Invoice Number is required.

When submitting order via Authorize.net HikaShop Plugin, we get the following error:

There was an error while processing your transaction: 33 (TESTMODE) Invoice Number is required.

I've reviewed documentation but unable to find resolution to issue in past Forum posts or HikaShop Documentation.

Gene Fourney
InteractiveWest