how does binary-amd64 work?

[Egon Willighagen <e.willighagen@science.ru.nl>]

Hi all,

I tried to install the deb for Jmol today from [1] which is Java software, so 
with my amd64 JVM (Blackdown) should work... so entered the apt sources

  deb http://debian.wgdd.de/debian testing main contrib non-free
  deb-src http://debian.wgdd.de/debian testing main contrib non-free

and did apt-get update...

But, this fails with messages like:

W: Couldn't stat source package list http://debian.wgdd.de testing/main 
Packages 
(/var/lib/apt/lists/debian.wgdd.de_debian_dists_testing_main_binary-amd64_Packages) 
- stat (2 No such file or directory)

What is the problem here? Isn't the binary-all in the repository good enough? 
Is there a reason why binary-amd64 should be present too?

Egon

1. http://debian.wgdd.de/