I can't send FB invitations.

Hello,

I have a problem with invitations on Facebook. I met someone and I would like to invite her to become my friend on Facebook, but when I request it, I get the following message: Invitations are intended for people you know, such as classmates, friends, family, or colleagues. To learn more, please visit our help pages. If you believe this message was shown in error, you can report the problem. There you go, and I only get this for her; when I ask someone else, it works. Thank you in advance for your help! ;D