This error can appear in random cases. The simple solution to fixing the problem is to close and restart CardExchange® Gateway.
If this error continues to occur even after restarting, open the card template causing the issue in the Card Designer. Ensure that all object of your card template are properly mapped in the properties window.
Another solution is to recreate the template. Try duplicating the template and edit a record. If the problem occurs, you may need to recreate the template from scratch.