Given this a Thing Planning register field, I believe you need to be referencing the "internal" register filed name (such as Code1, TermDate, etc), and not the user visible "alias" of the field. If you are unsure what the internal register field name is, go to Thing Planning Settings, and then Register Field List to see how the internal field names have been mapped to the visible field aliases.
If you are still in doubt, dump out the dictionary keys or keys and values found in args.NameValuePairs, as you are currently trying to access a dictionary key that does not exist in the dictionary.