Loot Pool Custom Data

I have a ui item determined by a loot pool. I want it to have minecraft:custom_data={"tag":"trade_station_ui"}. However every which way I try it, it doesn't work. I've tried "functions": [ { "function": "minecraft:set_custom_data", "tag": "\"tag\":\"trade_station_ui\"" } ] and "functions": [ { "function": "minecraft:set_custom_data", "tag": "{\"tag\":\"trade_station_ui\"}" } ] and even "functions": [ { "function": "minecraft:set_custom_data", "tag": "'{\"tag\":\"trade_station_ui\"}'" } ] The loot pool works fine until I add this (straight from the generator I might add), What am I doing wrong?