AMEX/Discover Auth.net not working

[webrob]

My Authorize.net account is setup for AMEX & Discover but I can not process them on my Website. I get the following error:

<img src="http://inkjetoutput.com/amex-error.jpg">

I am able to process these cards on the virtual terminal in the Authorize.net backend but on my website they get declined.

I have called Authorize.net and they say this is not one of their errors and I comes form the cart.

Any help is appreciated!