email address missing in unsubscribe admin notifications

We have been using the Subscribe by email plugin for a few weeks. When we get unsubscribes, they typically display the email address that is unsubscribing. In the last day we have received about 4 notifications that had no address.

They appear like:

Dear user,

A user () has removed from your subcribers list.

Any idea why this is happening? It would be good to know who is unsubscribing so we can track it in our other system.