Date Decision step error The contact field used for first, is BLANK, for the contact being processed

Some contacts are arriving to a date decision step in a program canvas and are being errored out with

{"Id":"522768","first":""} The contact field used for first, is BLANK, for the contact being processed

Why would we be seeing this error? I don't think this has anything to do with the First Name since each contact that errored out has a first name. What else could this field be?

Other contacts are processing through successfully.

Thanks!