Urgent! Problem with site in Safari

Hi there, my site is working fine in IE and FF but I've had a potential customer alert me that when they go to add a product to their shopping basket in SAFARI, they get the following error:

Bad Request

Your browser sent a request that this server could not understand.Client sent malformed Host header

I downloaded Safari and replicated this problem too. My site didn't show up any errors on a markup validation service...

I am using the default CSS template that I have modified.

I don't have this problem when I use the original default CSS template in Safari. So the problem is with my template but how can I begin to troubleshoot this and know where I have gone wrong???

Using v 1.3.8

Please help - many thanks in advance! My site is in my signature. Cheers!

I have copied my modified template onto another install of Zen cart and Safari works ok. So, could a mod be causing this?

I have copied my modified template onto another install of Zen cart and Safari works ok. So, could a mod be causing this?
You are following a good trouble shooting path