How to pass update=true and frames:v 1?

[fullmetalScience]

I would like to store a single image every second, overwriting the named file:

mpv --no-audio --vf=lavfi='[fps=1/60]' vid.mp4 -o img.png

The file does get written, but then mpv exits with this error:

    ● Video  --vid=1  (h264 1920x1080 30 fps) [default]
    [autoconvert] Converting yuv420p -> rgb24
    VO: [lavc] 1920x1080 rgb24
    [vo/lavc] Opening encoder: PNG (Portable Network Graphics) image [png]
    [encode] Opening muxer: image2 sequence [image2]
    [ffmpeg] image2: The specified filename 'img.png' does not contain an image sequence pattern or a pattern is invalid.
    [ffmpeg] image2: Use a pattern such as %03d for an image sequence or use the -update option (with -frames:v 1 if needed) to write a single image.
    [vo/lavc] detected an unexpected discontinuity (pts jumped by 59.999958 seconds)
    [ffmpeg] image2: Cannot write more than one file with the same name. Are you missing the -update option or a sequence pattern?
    [encode] Writing packet failed.
    V: 00:01:00 / 00:06:58 (14%) {0.1min 0.7fps 0.0MB}
    Exiting... (Interrupted by error)

How would I pass these options?