Can't Add Someone I Played With

I was playing with someone the other day and they were doing well, but I forgot to add them and got off. I ended up running into the person again in queue, but they dodged the game and I can't add them even though I know their summoner name. I think since it has some odd, non-english characters, it may be causing an issue. Their summoner name is Wûkõñg, however, when I try to add them it says they don't exist. This is the link to their OP.GG https://na.op.gg/summoner/userName=Wûkõñg. Any assistance would be appreciated.