Linkedin:recepient api request cannot be null

Question

I am trying to send an invitation, so first I look for the person using this:

Person profile = client.getProfileById(getThirdSocial("linkedin"));

then I get the person and I call the method： client.sendInviteToPerson(recepient, subject, message);

but I get an exception:

1-11 11:34:40.578: W/System.err(2697): java.lang.IllegalArgumentException?: recepient api request cannot be null.

recepient is not null because I am able to print out some details of the person. So I do not know what to do, please help.