GameCenter Sandbox rematch

I have the following code:

if(tappedItem.match.status == GKTurnBasedMatchStatusEnded){ 
NSLog(@"Participants %@", [tappedItem.match.participants description]);
  [tappedItem.match rematchWithCompletionHandler:^(GKTurnBasedMatch *match, NSError *error) {
if (error) { NSLog(@"%@", error);
} else {
[[GameKitHelper sharedGameKitHelper] setMatch:tappedItem.match];
[[NSNotificationCenter defaultCenter] postNotificationName:ShowGameScreen object:tappedItem.match]; 
}
}]; 
}

I've got a number of people Beta testing it via TestFlight with sandbox enabled, but for some reason I get the following error when trying to rematch:

{GKServerStatusCode=5121, NSLocalizedDescription=The requested operation could not be completed because the player is invalid., NSUnderlyingError=0x17045cdd0 "The operation couldn’t be completed. status = 5121, Invitation from: 224002977 to: 225851510 is not allowed because they are neither friends nor have recently played"}

The match was ended properly, so it's not that:

[_match endMatchInTurnWithMatchData:data scores:scores achievements:nil completionHandler:^(NSError *error) {}];

I'm thinking this is an isolated issue with Sandbox, but unless I can test it I'm not confident that it will work in the once released.