I am trying to create a base map projection and I want to filter out Antarctica. I have tried this but it is not working:

import pandas as pd
import numpy as np
import altair as alt
from vega_datasets import data
import urllib
import json
import requests

#import topoJSON
url = "https://cdn.jsdelivr.net/npm/world-atlas@2/countries-50m.json"
countries = alt.topo_feature(url, "countries")

# World map
world_map = alt.Chart(countries).transform_filter(
    alt.datum["properties.name"] != "Antarctica").mark_geoshape(
    fill='lightgray',
    stroke='white'
).encode(
    color=alt.value("lightgray")
).project(type="mercator")

Any help is greatly appreciated. Thank you!