You’ll need to make sure you’re referencing the correct internal property names. Relating to your example, HubSpot’s standard internal names for these properties are “firstname” and “lastname” (i.e. not separated by underscore). You might like to consider reviewing HubSpot’s Properties API documentation. You can also find the internal names within HubSpot’s web UI via Settings > Properties > {Property} > “</>” symbol at top-right of screen.