What does -no PR in Long Tail Pro mean?

I'm getting the hang of keyword research using LTP, but I want to know what the result "-no PR" means? There's a result for "0" page rank, and then there's one that says "-no PR". Does that mean it has no page rank? Is there a different between PR 0 and No PR? I found a keyword with a bunch of No PRs, but I want to know what it means for sure before I move on and buy the domain.

Thanks.

Anyone know?

Its mean PR not available and less than 0 (Usually for new content).