sort:index was meant to be a shortcut for sort:"index asc", but this currently causes a NPE.
it's probably because facet.sortVariable is not set if there is no direction set. The patch is uploaded. There is a test added in patch to catch this failure.
Commit 1714641 from Yonik Seeley in branch 'dev/trunk'
[ https://svn.apache.org/r1714641 ]
SOLR-8284: fix NPE short form sort:index and sort:count
Commit 1714642 from Yonik Seeley in branch 'dev/branches/branch_5x'
[ https://svn.apache.org/r1714642 ]
Committed. Thanks Michael!