Redemption fails with "unicode has no attribute number" exception

[_zkapauthorizer.controller.PaymentController#error] Redeeming random tokens for a voucher (TTqL5_e35TsSSCAoWlR1Osy-nByHDDEfiKM-2SKnMB9o) failed: [Failure instance: Traceback: <type 'exceptions.AttributeError'>: 'unicode' object has no attribute 'number'
2020-04-30 09:43:53,718 DEBUG line_received [<Filtered:GatewayName:1>] >>> 	_zkapauthorizer/controller.py:730:redeem
2020-04-30 09:43:53,718 DEBUG line_received [<Filtered:GatewayName:1>] >>> 	_zkapauthorizer/controller.py:686:_perform_redeem
2020-04-30 09:43:53,718 DEBUG line_received [<Filtered:GatewayName:1>] >>> 	twisted/internet/defer.py:1613:unwindGenerator
2020-04-30 09:43:53,718 DEBUG line_received [<Filtered:GatewayName:1>] >>> 	twisted/internet/defer.py:1529:_cancellableInlineCallbacks
2020-04-30 09:43:53,718 DEBUG line_received [<Filtered:GatewayName:1>] >>> 	--- <exception caught here> ---
2020-04-30 09:43:53,718 DEBUG line_received [<Filtered:GatewayName:1>] >>> 	twisted/internet/defer.py:1418:_nlineCallbacks
2020-04-30 09:43:53,718 DEBUG line_received [<Filtered:GatewayName:1>] >>> 	_zkapauthorizer/controller.py:847:bracket
2020-04-30 09:43:53,718 DEBUG line_received [<Filtered:GatewayName:1>] >>> 	]
2020-04-30 09:46:51,598 DEBUG line_received [<Filtered:GatewayName:1>] >>> 2020-04-30T09:46:51-0400