Excel JS WebImages Sample code produces error on valuesAsJson

Question

When I run the sample code from ScriptLab on InsertImage:

async function insertImage() {
  // This function inserts a web image into the currently selected cell.
  await Excel.run(async (context) => {
    // Retrieve image data from the task pane and then clear the input fields.
    const imageUrl = $("#url").val() as string;
    const imageAltText = $("#alt-text").val() as string;
    clearForm();

    // Load the active cell.
    const activeCell = context.workbook.getActiveCell();
    activeCell.load();
    await context.sync();

    if (!imageUrl) {
      console.log("Please enter an image URL.");
      return;
    }

    // Create a web image object and assign the image details.
    const webImage: Excel.WebImageCellValue = {
      type: "WebImage" /* The string equivalent of `Excel.CellValueType.webImage`. */,
      address: imageUrl,
      altText: imageAltText
    };

    // Insert web image into the active cell.
    activeCell.valuesAsJson = [[webImage]];

    await context.sync();
  });
}

I get an error revolving around valueAsJson. I am able to load the image directly with window.open(UrlAddress, "_blank"); so there is nothing wrong with the URL.

code: "GeneralException"
message: "There was an internal error while processing the request."
toString: function toString()
errorLocation: "Range.valuesAsJson"

What am I doing wrong?