set_lore Item Modifier not working with storage source

I have an item modifier which looks like this:
{
    "function": "minecraft:set_lore",
    "lore": [
      [
        {
          "text": "Value: ",
          "italic": false
        },
        {
          "nbt": "path.value",
          "storage": "namespace:storage"
        }
      ]
    ],
    "mode": "append"
  }

I'm trying to set the lore to show the value from the storage (an integer).
When I apply it, it sets the "Value: " part as lore, but the storage bit is left blank.
I have verified that the storage does in fact contain the correct value when the item modifier is applied.
Any idea what could be going wrong?