Last name of Recommender.

In the section where I am required to add a recommender, I have a professor who has only provided a first name. However, the application form mandates the inclusion of a last name for the recommender. Unfortunately, my recommender does not have a last name.

Could anyone please advise me on how to proceed in this situation? Should I use a specific placeholder like "N/A" or "None"

Or if anyone has encountered this situation, then how did he/she tackle this.

Thanks in advance.