Fatal Error on Import with External Identifier

The bug occurs under these conditions:

1.) Import matching method is set as "Update", AND
2.) Import data file contains External Identifier field to be mapped, AND
3.) Import data file contains ANY contacts that do not yet exist in CiviCRM.

Rather than simply adding new contacts, the import process Fails with an 'unexpected error' as soon as it reaches a row containing an unknown External Identifier.

Sample Data attached, as used for testing on demo site. Import the contacts (without matching) in the first file, then attempt to update & add contacts by importing the second file.