Outputting very short clip fails

[dubhater]

test.lua:

vs = require "luasynth"
blank = vs.std.BlankClip{length=1}
blank:output(io.stdout, false)

luajit test.lua > frame.raw can result in:

• "No frame returned at the end of processing by BlankClip"
• "Frame returned not of the declared type"
• Segmentation fault in libvapoursynth.so or libstdc++.so

length=2 or larger works.
length=1 and prefetch=1 also works.
length=2 and prefetch=3 breaks.
length=3 and prefetch=3 also works.
I have two CPU cores with no hyperthreading.