how to add a description element?

[rsignell-usgs]

I'm writing a script to convert bird migration data in CSV files to CZML. It's working well so far, but I'd like to add a description element as per this suggestion from Kevin Ring:
TerriaJS/terriajs#1275 (comment)

When I try to add this to my script here:
https://gist.github.com/rsignell-usgs/d0b1fd95f79204a50814#file-bird2czml-py-L110
I get no errors, but the output CZML file doesn't contain the description.

I looked at the code and it seems like there is some description stuff in there:
https://github.com/cleder/czml/blob/master/czml/test_main.py#L1015-L1017
but I couldn't figure out how to use it.

Is the description element actually implemented?
If so, how do I use it?

[rsignell-usgs]

Here's a more clear example that demonstrates what I tried, but doesn't work:
https://gist.github.com/rsignell-usgs/1ba00682610dd63a8bd5
(screen grab here):

This is frustrating because it looks like there is a test here that demonstrates this working:
https://github.com/cleder/czml/blob/master/czml/test_main.py#L1039-L1046

What am I doing wrong?