Exception thrown when fields without values are used in mappings

I have the following situation:

Entities exist, and are tied to Mailchimp Lists using a mailchimp field. I add a new field to these entities for an Address: the existing entities have no data in the database for this new field. I add the Addressfield sub-values to the Mailchimp merge fields in the Mailchimp field configuration.

Now, any time I try to edit and save one of the old entities, I get a MetadataWrapperException because Mailchimp Lists is trying to compare the old and new values in "_mailchimp_lists_subscription_has_changed()", and the old item has no value to get.