You know, if you're so neurotic about being a Good Ally that you're in a nonstop state of flop-sweating panic about being accidentally racist, maybe it's time to take a deep breath, count to ten, and remind yourself that this isn't rocket science. It's supposed to be natural.
I used to teach ESL in Los Angeles and had tons of Mexican students and friends. This exact thing happened to me a couple times, and do you know what happened? We laughed. And moved on. Because it's not a big deal. You did not Erase Spaces like you snapped the Infinity Gauntlet and suddenly your bosses' identity disintegrated.
Frankly, whenever someone talks about not being racist as if it's some exhausting chore, alarm bells start ringing in my head. Going out of your way to understand people who are different from you--racially, culturally, geographically, philosophically, whatever--isn't a holy crusade for a righteous few, it's a very, very basic component of social interaction and personal growth. Children can do it effortlessly.
It should not be work.
If this is something you're struggling with, having a nervous breakdown because you assumed a person with white skin was of European ancestry, you really need to step back, look at the big picture, and remind yourself that freaking out is not helping anybody.