I'm not sure what version of Excel or calculator you're using. But 0/10 is 0. It's the reverse 10/0 that is not valid.
For that single round 0 kills and 10 deaths is a k/d of 0. But for your cumulative stats you add the 10 to whatever your current total deaths are. So it could still bring your career k/d down even futher. Unless you've somehow never had a kill in the game. In which case it would remain 0 for the career as well.
Example:
Total all-time kills is 100. Totall all time deaths is 200. Career k/d is .5 (100/200). Then you go 0/10 in the next round. Your total kills remains 100. But your total deaths is now 210. So your career k/d goes down to .48 (100/210).