3D Scatter X's not rendering properly with image.save_as

I have a 3d scatter plot I need to render a series of images from. Calling image.save_as works perfectly with the exception of the "X"s on my scatter plot. These show up fine within the iPython notebook, but when I call image.save_as, my X’s become boxes. Here’s the relevant code I’m calling:

negatives = Scatter3d(
    x = [0],
    y = [1],
    z = [1], 
    mode = 'markers',
    marker = dict(size = 6, symbol = 'x', color = 'red'),
    showlegend = False
)

And my plotting code:

data = Data([negatives])
layout = Layout(autosize = False, width = 1000, height = 800)
fig = Figure(data=data, layout = layout)