I am seeing this bug also: it has been persisting for several months, but I only just got around to reporting it.
I have librunby2.5, version 2.5.7-1. This is on testing updated most days. # /usr/sbin/dhelp_parse -i index++: error: could not read index from "/var/lib/dhelp/documents.index": No data available Dhelp::IndexerError: Couldn't index using /usr/bin/index++ --config-file /usr/share/dhelp/config/swish++.conf --index-file /var/lib/dhelp/documents.index --follow-links --incremental - (/usr/lib/ruby/vendor_ruby/dhelp.rb:616:in `index' /usr/sbin/dhelp_parse:171:in `do_deferred_indexing' /usr/sbin/dhelp_parse:205:in `main' /usr/sbin/dhelp_parse:221:in `<main>')