title: UK geojson displays blank folium map
body: <p>I'm trying to use the following geojson for UK local authorities, but it displays a blank map.</p>
  <p>&lt;https://martinjc.github.io/UK-GeoJSON/json/eng/topo_lad.json -O UK_counties.json&gt;</p>
  <p>My table lists most of the local authorities in the same way. Not sure if its the key_on reference that's the only problem or if there's more to it.</p>
  <p>My code so far is below:</p>
  <pre><code>threshold_scale = np.linspace(df_ctbl['Population'].min(), df_ctbl['Population'].max(), 6, dtype=int)
  
  threshold_scale = threshold_scale.tolist() # change the numpy array to a list
  threshold_scale[-1] = threshold_scale[-1] + 1 # make sure that the last value of the list is greater than the maximum immigration
  
  # let Folium determine the scale.
  londonf_map = folium.Map(location=[latitudeld, longitudeld], zoom_start=4, tiles='Mapbox Bright')
  londonf_map.choropleth(
      geo_data=uk_geo,
      data=df_ctbl,
      columns=['Local Authorityp', 'Population'],
      key_on= 'properties.lad13nm',
      threshold_scale=threshold_scale,
      fill_color='YlOrRd', 
      fill_opacity=0.7, 
      line_opacity=0.2,
      legend_name='UK Population in 2019',
      reset=True
  )
  londonf_map
  
  </code></pre>
  <p>Can anyone help? Thanks</p>