magento invoice exists but has no id

Question

I made a custom module with observer: and everything works fine except the invoice ID. The invoice id remains empty. I tried the following code:

$event = $observer->getEvent();
$invoice = $event->getInvoice();
$order = $invoice->getOrder();

Mage::log($invoice->getId(), null, "test.txt");
Mage::log($invoice->getEntityId(), null, "test.txt");
Mage::log($invoice->getData("entity_id"), null, "test.txt");

It keeps printing an empty string for the invoice ID.

What am i doing wrong here?