It's unfortunate that you were not expecting this. First, check your spam folder — it can happen that the invoice accidentally ends up there.
Are you also looking in the correct mailbox? The invoice is sent to the email address known to us before the direct debit. For example, if you registered with your work email address, you might not find it in your personal inbox.
Can't find the invoice anywhere? Then please contact us, and we will resend the invoice to the email address known to us.