Errors in Structured Data - Missing "fn"

Hi, hoping there’s a solution to this. I’m getting many errors in Google Webmaster. The message is “missing fn” – hcard. So if a post has 100 comments, I get 100 errors. Below is a sample message in Webmaster:
hcard
itemtype:
http://microformats.org/profile/hcard
photo:
http://2.gravatar.com/avatar/efcf9ef7b5ec55c40f337be6b63d53ed?s=64&d=retro&r=g

This error is repeated for each gravatar in every comment.