Notice: Undefined index: !username in heartbeatParser->create_gap_id() (line 297 of /var/www/mujo_demo/champions/sites/all/modules/contrib/heartbeat/includes/heartbeatparser.inc).

I sometimes get this error. After refreshing there is no more error, but after loading some other pages it's there again.

I've googled the error, but i couldn't find any solution.

Is there someone who can tell me why I'm getting this error?

Thanks.