Error after mass subscribe contacts to an event

After a mass subscribe and clicking 'Save and new' an error occured:

"cid (value: ) is not of the type Positive"

Detail:

• search contacts with advanced search
• select the contacts I want to subscribe
• clicking 'Add contacts to event'
• fill the form (selecting event etc.)
• clicking 'Save and New'
• error: "cid (value: ) is not of the type Positive"

Remarks:

• despite the error, the contacts are subscribed to the selected event
• after clicking 'save' there is no error