Using `grab_frame` with `FFMpegFileWriter` Yields Error in the Figure Argument to Subprocess

Question

I want to generate an MP4 file using frames produced by a rather complicated simulation. If I have to watch the movie as a side affect of generating it I will not complain too much, but would rather not "show" anything on the screen. The overhead of making the movie is trivial compared to the simulations, so I care much more about clean simple code than about performance.

It seemed natural to use grab_frame with FFMpegFileWriter. The following code seemed right.

fig, ax = plot.subplots(figsize=(6.5, 6.5))

assert(animation.FFMpegFileWriter().isAvailable())
writer = animation.FFMpegFileWriter(fig)
writer.setup(fig, "test1.mp4",224)

with writer.saving(fig,"test1.mp4", dpi=224):
    for i in range(len(frameSeg)):
        PlotFrame(i)
        writer.grab_frame()

# writer.finish()

print("done")

It runs, but yeilds the following error

[image2 demuxer @ 0000021fab8c1000] Unable to parse option value "Figure(650x650)" as video rate [image2 demuxer @ 0000021fab8c1000] Error setting option framerate to value Figure(650x650). [in#0 @ 0000021fab8c1e00] Error opening input: Invalid argument Error opening input file ... AppData\Local\Temp\tmpe19ci2wz\tmp%07d.png. Error opening input files: Invalid argument

subprocess.CalledProcessError: Command '['ffmpeg', '-framerate', 'Figure(650x650)', '-i', 'F:\Users\Kenne.DESKTOP-BT6VROU\AppData\Local\Temp\tmpe19ci2wz\tmp%07d.png', '-loglevel', 'error', '-vcodec', 'h264', '-pix_fmt', 'yuv420p', '-y', 'test1.mp4']' returned non-zero exit status 4294967274.

Did I miss something in setup, is it an installation problem, or is it a bug?

By the way using FFMpegWRiter with FuncAnimation works ...