Undefined offset: 0 in crm_core_contact_label

In version 7.x-1.0-beta1+105-dev I get the following notice when I add an event:
Undefined offset: 0 in crm_core_contact_label() (line 514 of xxx/sites/all/modules/contrib/crm_core/modules/crm_core_contact/crm_core_contact.module).

At first I create an event page. Everytime I add only one participant on this page, the program adds one or more empty participants. When I go to the registration tab of the page and try to edit an empty registration I get this error message.
In the contacts these empty "participants" are added as well. When I delete these contacts I get the same notice.