CZML Rectangle with ImageMaterial over time doesn't work

CesiumJS
1

Based on what I’ve seen in some older posts, if I want an image material to vary over time, I set it up like this: (assume the uri would differ)

 "material": {
    "image": {
      "image": {
        "uri": [
          {
            "interval": "2020-04-23T00:10:19Z/2020-04-23T00:10:29Z",
            "uri": "https://sandcastle.cesium.com/CesiumUnminified/Assets/Images/ion-credit.png"
          },
          {
            "interval": "2020-04-23T00:10:29Z/2020-04-23T00:10:39Z",
            "uri": "https://sandcastle.cesium.com/CesiumUnminified/Assets/Images/ion-credit.png"
          },
          {
            "interval": "2020-04-23T00:10:39Z/2020-04-23T00:10:49Z",
            "uri": "https://sandcastle.cesium.com/CesiumUnminified/Assets/Images/ion-credit.png"
          },
          {
            "interval": "2020-04-23T00:10:49Z/2020-04-23T00:10:59Z",
            "uri": "https://sandcastle.cesium.com/CesiumUnminified/Assets/Images/ion-credit.png"
          }
        ],

Except, it doesn’t work–it throws an error to the console:

DeveloperError: julianDate is required.
(stack cut because it won't let post all those links)

Poking around in the source for a bit, it would seeeeem that it thinks that it’s an interpolatable property (which the docs say it’s not). But If I add an epoch, then I get this:

TypeError: innerType.unpack is not a function
(stack snip)

Here’s a sandcastle with this reproducible. Am I just missing something?