not valid rss feed if using category limits

hello dear friends,
i am using wp-2.05 atm, and i am generating a rss feed, which should contain only two categories. it works pretty well, but valdidome tells me, that it is unvalid code. what should i do?
tag: link rel=”alternate” type=”application/rss+xml” title=”Backlab Collective RSS Feed” href=”https://www.backlab.at/hp2006/?feed=rss2&cat=2,3″

adress: https://www.backlab.at/hp2006/

error message:

Line Column: 133
8 Error: general entity “cat” not defined and no default entity
Error Position:

…ctive RSS Feed” href=”https://www.backlab.at/hp2006/?feed=rss2&cat=2,3″ />

Column: 136
Error: reference to entity “cat” for which no system identifier could be generated
Error Position:

…ctive RSS Feed” href=”https://www.backlab.at/hp2006/?feed=rss2&cat=2,3″ />

Column: 132
Error: entity was defined here
Error Position:

…ctive RSS Feed” href=”https://www.backlab.at/hp2006/?feed=rss2&cat=2,3″ />

thanx for your help in advance!