NcDonald's?

My last name begins with "Mc." (Let's fictionalize it into McDonald for now.)

Three times over the past few weeks, when giving personal information out over the phone, I've had people on the other
end of the line confound me with their confusion over the spelling of my
last name. I'll go to spell it: "M...c....capital D...o..." and they'll interrupt me - "Are you saying 'M' or 'N'?" And I stop, confused.

Am I saying "N?" How many last names start with Nc? NcDonald's? NcNulty? NcIntyre? Who are these people, and why do they always make me feel like an idiot who's misspelling my own name?