Using cartopy + digitalocean

I use cartopy in a python script to periodically generate images of earth and tweet them. My code to generate the image is essentially:

import matplotlib.pyplot as plt
import cartopy.crs as ccrs

ax = plt.axes(projection=ccrs.NearsidePerspective(central_longitude=long, central_latitude=lat, satellite_height=35785831/3))
ax.stock_img()
plt.savefig("earth.png")

If I run my code from my local Windows machine, the image is tweeted out correctly, whereas on digitalocean, it does not insert the stock image (from ax.stock_img(). The versions of cartopy should be identical, the image is on both machines in the same place.

I’m not sure if this is something the DO community can help with (I hope it is!) but has anyone encountered this before and if so – how do I rectify it?