pandoc filter does not read metadata - not a valid json value

问题

The filter copied from https://pandoc.org/filters.html and run with

pandoc -f markdown  -t markdown  --filter filterUmlaut -o test1.md coronatest.md

produces the error

Failed reading: not a valid json value at 'input---ntitle:TheFutureofAAn...nn##secondlevelntextfornothingn'

which is the metadata at the beginning of coronatest.md. The file can be run with the same command without the filter option:

pandoc -f markdown  -t markdown   -o test1.md coronatest.md

What do I miss?

来源：https://stackoverflow.com/questions/62060820/pandoc-filter-does-not-read-metadata-not-a-valid-json-value