Please advise, but Grammar Girl opines that the problem here is 'false attraction to a predicate noun'. I excerpt the fundaments:
Although this problem may seem complicated, it’s really not. It’s as simple as this: the verb agrees with the subject (2)1, not the predicate noun. - See more at: Verbs sandwiched between singular and plural nouns - Quick & Dirty Tips
Dorothy, don’t pay attention to the man behind the curtain, meaning don’t be distracted by the predicate noun. One grammar source calls this problem “false attraction to a predicate noun” (3). - See more at Quick & Dirty Tips
1. The (2) and (3) refer to external references on p 2 of 2 of Grammar Girl's article.