What is the correct syntax for checking CustomName nbt in a loot table condition

I have 2 different "types" of zombie, told apart by different CustomName tags assigned at spawn. I want them to have different loot tables too, but my current code simply removes all drops from zombies instead.

(file structure and the rest of the code is correct because it works when I’m not checking CustomName)

Example: "Drop gold if zombie is named "I-drop-gold"

{
  "type": "minecraft:entity",
  "pools": [
    {
      "rolls": 1,
      "entries": [
        {
          "type": "minecraft:item",
          "name": "minecraft:gold_ingot",
          "functions": [
            {
              "function": "minecraft:set_count",
              "count": 1
            }
          ]
        }
      ],
      "conditions": [
        {
          "condition": "minecraft:entity_properties",
          "entity": "this",
          "predicate": {
            "nbt": "{CustomName:I-drop-gold}"
          }
        }
      ]
    }
  ]
}

Why doesn’t this work, what is the correct formatting?