Mensch is masculine, but I saw ein ehrlicher Mensch today. I have seen similar cases, but don’t understand why one must use ein.
to suggest your expected article: "der Mensch", instead of "ein Mensch". I can summarize the above comments for you: neither "der Mensch" nor "ein Mensch" is wrong. As "der" is the definite masculine it refers to a specific individual human (or in this case it can mean humanity as well but let's forget about that). "ein" refers to one but any human.
As I already mentioned your case is special because of the overladed meaning of "Mensch". So let's take it by "Person": The person, who ate my sandwich, will pay for that sin! - "Die Person" - because it is not any person but one special person. I need a person to hunt the sandwich-thief down. - "Eine Person" - because any person is sufficient.