I've read almost every post related to this issue but still can't resolve for my site. For whatever reason, authnet is expecting an MD5Hash even though I don't have one set up for my cart? Zencart version:

Zen Cart 1.3.8a
Patch: 1::
Database Patch Level: 1.3.8
v1.3.8 [2009-11-19 10:35:58] (Fresh Installation)

No addons installed
Hosting company is GoDaddy

Authorize.net(AIM) Configs:

Enable Authorize.net (AIM) Module
True

Login ID
*****

Transaction Key
****************

MD5 Hash


Transaction Mode
Production

Authorization Type
Authorize+Capture

Enable Database Storage
True

Customer Notifications
True

Merchant Notifications
True

Request CVV Number
True

Sort order of display.
0

Payment Zone
--none--

Set Completed Order Status
Processing [2]

Set Refunded Order Status
Pending [1]

Debug Mode
Log and Email

Cut from log file:
[Expected-MD5-Hash] => 854211C7C2808D8850088BD9E64FD179
[HashMatchStatus] => FAIL
)