Unable to use the /tag command to tag if wielding a specific item [duplicate]

Me and my friends have been creating a Minecraft realm, and I figured that I would make custom bosses and custom drops to spice up the game a bit. I've just got to a weapon that I'm trying to cause wither in other players when it hits, but first to do that I'm trying to tag the wielder. I wrote the command /tag @e[nbt={SelectedItem:{id:"minecraft:iron_sword",tag:{display:{Name:"Sword of Iapetos"}}}}] add IapetosSword

For some reason though, whenever I hold the sword, it doesn't tag me! I assume I wrote the code wrong, but I can't figure out where.

NBT checks must be exact and item names are JSON, not simple strings. So instead of …

"Sword of Iapetos"

… check for:

"{\"text\":\"Sword of Iapetos\"}"

Or alternatively:

'{"text":"Sword of Iapetos"}'

You can find out things like this easily by running this command:

/data get entity @s SelectedItem