No file output produced by melt

Question

I am using melt (based on MLT) to produce a gif with transitions from three images, as per Lordneckbeard's answer in this QA.

melt -verbose \
Screenshot_20180514-172311.png out=50 \
Screenshot_20180514-172231.png out=75 -mix 25 -mixer luma \
Screenshot_20180514-172252.png out=75 -mix 25 -mixer luma \
-consumer avformat:anim.gif 

This produces the video as intended, shown via SDL. However, no output is written to disk. This happens even if other formats are used, for example x264 as per the linked QA above. Moving the -consumer argument to the top* has no effect.

How can I get melt to produce file output?

^{* I wondered if order of operators was important, as in ffmpeg}